RAF jets were scrambled to intercept four Russian military aircraft to the north of Scotland.
Typhoons from RAF Lossiemouth in Moray and a Voyager fuel tanker from RAF Brize Norton in Oxfordshire were involved in the mission.
The RAF said the Russian aircraft did not enter UK airspace.
Russia has condemned a US decision to send extra troops to Europe to support its Nato allies amid continuing fears of a Russian invasion of Ukraine.
Moscow said it was a "destructive" step which heightened tension and reduced the scope for a political solution.
The Pentagon said 2,000 US troops would be sent from North Carolina to Poland and Germany, and a further 1,000 already in Germany would go to Romania.
Russia has some 100,000 troops near Ukraine. It denies planning to invade.
The tensions come eight years after Russia annexed Ukraine's southern Crimea peninsula and backed a bloody rebellion in the eastern Donbas region.
Moscow accuses the Ukrainian government of failing to implement the Minsk agreement - an international deal to restore peace to the east, where Russian-backed rebels control swathes of territory and at least 14,000 people have been killed since 2014.
Responding to US President Joe Biden's decision to deploy extra troops to Europe this week, Russian Deputy Foreign Minister Alexander Grushko said it was a "destructive" and an "unjustified" step.
Speaking on Wednesday, Mr Grushko added that it would "delight" the Ukrainian authorities, who would continue sabotaging the Minsk agreement "with impunity".
The Pentagon earlier said the American troops being deployed would not fight in Ukraine - but would ensure the defence of Washington's allies.
Their deployment is in addition to the 8,500 troops the Pentagon put on alert last month to be ready to deploy to Europe if needed.
"It's important that we send a strong signal to Mr Putin and, frankly, to the world that Nato matters to the United States and it matters to our allies," Pentagon spokesman John Kirby told reporters on Wednesday.
But on the question of alleged invasion plans by Mr Putin, he said: "We still don't believe he's made a decision to further invade Ukraine."
He also said a US proposal "leaked to a European news outlet" was genuine. He appeared to be referring to a story in Spain's El País newspaper about a US offer of talks with Russia on cutting back on nuclear weaponry and on trust-building measures in exchange for reducing tensions over Ukraine.
Meanwhile, intense diplomatic efforts continued to try to find a political solution to the crisis, amid fears it could blow up to a full-scale war in Europe.
Russian President Vladimir Putin discussed Ukraine with UK Prime Minister Boris Johnson by phone on Wednesday.
Mr Johnson told Mr Putin "all European democracies have a right to aspire to Nato membership", a Downing Street statement said. Russia opposes Ukraine joining the security alliance.
Mr Putin drew attention to Nato's reluctance to respond appropriately to Russia's justified concerns, the Kremlin said in a statement (in Russian).
The Russian president earlier accused the US of trying to draw his country into a war in Ukraine. He said America's goal was to use a confrontation as a pretext to impose more sanctions on Russia.
Separately, President Biden and his French counterpart Emmanuel Macron "affirmed their support for Ukraine's sovereignty and territorial integrity," the White House said in a statement.
In a phone call, they reviewed "ongoing co-ordination on both diplomacy and preparations to impose swift and severe economic costs on Russia should it further invade Ukraine," the statement said.
These are combat-ready troops but they are not expected to be fighting Russian soldiers - if they were the deployment would be much bigger. After all Moscow is not seen to be directly threatening Poland and Romania.
Joe Biden is trying to show this resolve without undermining chances of a diplomatic solution. He is not sending a lot of troops, and notably, none to the Baltic states right on Russia's doorstep.
But these deployments are the most concrete sign that the current crisis is broader than Ukraine, that it's over the post-Cold War security infrastructure in Europe.
Rivalry between Russia and the US, which still possess the world's biggest nuclear arsenals, dates back to the Cold War (1947-89). Ukraine was then a crucial part of the communist Soviet Union, second only to Russia.
New York has introduced new restrictions aimed at curbing coronavirus, with Mayor Bill de Blasio warning it was the city's "last chance" to stop a second wave.
Bars, restaurants and gyms must close by 22:00 and people can only meet in groups of 10 or less.
The US is seeing a surge in coronavirus - a record 61,964 Americans were in hospital on Tuesday.
An average of over 900 people a day are now dying with the disease.
More than a million new cases in November pushed the total confirmed cases to over 10 million nationally with 239,732 deaths so far.
The US has been seeing more than 100,000 new cases per day over the last 10 days in what experts say may be a worse outbreak than those seen in the spring and summer.
Experts warn hospitals across the country could soon be overwhelmed.
On Wednesday a member of President-elect Joe Biden's Covid-19 advisory panel said a four to six week lockdown could bring the pandemic under control.
Dr Michael Osterholm said that the government could borrow enough money to cover lost income for businesses during a shutdown.
"We're seeing a national and global Covid surge, and New York is a ship on the Covid tide," state Governor Andrew Cuomo said on Wednesday.
New measure come into effect on Friday affecting hospitality after Mr Cuomo said contact tracing identified late-night gatherings as key virus spreaders in the state.
If the rate of spread of infection continued to rise, Mayor Blasio said the New York City's public school system would close and children would begin online classes.
"This is our last chance to stop a second wave. We can do it, but we have to act now," Mr de Blasio tweeted.
New York City was badly hit by the virus earlier this year when nearly 18,000 people died with Covid-19 in March, April and May, according to the city's Department of Health and Mental Hygiene.
States across the US have broken new case records this week. On 10 November, Texas became the first state to hit one million total cases. If Texas were a separate country, it would rank 11th in the world for most cases.
Other states, including Illinois, Wisconsin, Minnesota, California and Florida, have also seen numbers rise. CBS News reports 15 states saw the numbers of patients in hospital due to the virus double in the last month.
Some hospitals, such as in Idaho and Missouri, have had to turn patients away because they ran out of room.
State leaders have been re-imposing pandemic restrictions as a result. Residents of Wisconsin and Nevada have been urged to stay at home for two weeks. In Minnesota, bars and restaurants must shut by 22:00.
On Tuesday, Mr Osterholm warned of a "perfect storm". Speaking to the CBS This Morning programme, Mr Osterholm said there was "no question that our hospitals are about to be overrun". He noted "the darkest days of this pandemic between now and next spring", before the vaccine arrives.
Mr Osterholm, who heads the infectious disease research centre at the University of Minnesota, said during the summer spike after the Labour Day national holiday, new cases rose to 32,000 a day.
"Now we're running in the 120- to 130,000 cases a day," he said. "Do not be at all surprised when we hit 200,000 cases a day."
The same day, US infectious disease chief Dr Anthony Fauci offered some hopeful news. He said the new Covid vaccine by Pfizer was expected to go through an emergency authorisation process in the next week or so. Human trials suggest it is 90% effective.
Dr Fauci told MSNBC: "I'm going to look at the data, but I trust Pfizer, I trust the [Food and Drug Administration]. These are colleagues of mine for decades, the career scientists."
Amid the ongoing outbreak, the US Centers for Disease Control and Prevention (CDC) has updated its research around masks, saying that wearing one not only protects others but also the person wearing the mask.
Previous guidance had rested on the idea that the main benefit of mask-wearing came from potentially stopping an infected person transmitting Covid to others.
The CDC referenced several studies, including one case where two Covid-positive hair stylists interacted with 139 clients - but of the 67 clients researchers tested, none developed an infection. The stylists and all clients had worn masks in the salon.
Another study looking into the outbreak aboard the UUS Theodore Roosevelt aircraft carrier found mask-wearing seemed to have reduced the risk of virus transmission by 70%, the CDC said.
Brazil's health regulator has announced that the trial of a Chinese coronavirus vaccine can resume.
Anvisa suspended Brazil's trial of the vaccine two days ago, citing a "severe adverse incident" - reported to be the death of a volunteer.
The head of the institute conducting the trial said the death had no connection to the vaccine.
In a statement on Wednesday, Anvisa said it now had "sufficient information to allow vaccination to resume".
"It is important to clarify that a suspension does not necessarily mean that the product under investigation does not offer quality, safety or efficacy," it said.
President Jair Bolsonaro earlier declared the suspension a "victory".
He has long criticised the vaccine because of its Chinese links and said it would not be purchased by his country. He has also engaged in a political fight with the governor of São Paulo, João Doria, who has publicly backed the trial.
Mr Bolsonaro has not yet commented on Anvisa's announcement that the trial can resume.
The vaccine, developed by Chinese firm Sinovac Biotech, is one of several in final-stage testing globally. Sinovac says it is "confident in the safety of the vaccine".
Brazil has been one of the countries worst affected by coronavirus, recording more than 5.6m confirmed cases - the third highest tally in the world after the US and India - and nearly 163,000 deaths, according to data collated by Johns Hopkins University.
On Monday, Anvisa said it had "ruled to interrupt the clinical trial of the CoronaVac vaccine after a serious adverse incident".
It did not reveal what had happened, nor where it had taken place.
Dimas Covas, head of the Butantan institute conducting the trials, told local media that the trial's suspension was related to a death, but insisted that the death was not linked to the vaccine.
This was backed up by Jean Gorinchteyn, health secretary for the state of São Paulo, who told a news conference that the death was an "external event" that was not related to the vaccine.
Mr Covas said that there had been no adverse reactions to the vaccine, and that the decision to suspend the trial had caused "indignation".
Media reports say police are investigating the death as a suicide.
In its statement allowing the resumption of the trial on Wednesday, Anvisa said it now had more details on the nature of the "adverse incident".
It said its decision to suspend the trial "took into consideration the data known to the agency at the time".
A pause in a clinical trial is not unusual. In September, the UK paused trials for another Covid-19 vaccine after a participant had a suspected adverse reaction.
The trials for the vaccine being developed by AstraZeneca and Oxford University were resumed a few days later after regulators said it was safe to continue.
In October, a review into the death of a Brazilian volunteer involved in the AstraZeneca trial found no safety concerns. The BBC understands the volunteer did not receive the vaccine.
Brazil's president has been open about his preference for the vaccine being developed by AstraZeneca, saying his government would not buy a Chinese-made Covid-19 vaccine.
