Planes monitor Russian bombers
British fighter planes and a warship were scrambled as Russian bombers and vessels passed close to the UK.
First it emerged that HMS Argyll had been deployed to monitor a destroyer and two other ships from the country as they passed through the English Channel.
Hours later the Ministry of Defence (MoD) disclosed that Typhoons were sent from RAF Lossiemouth in Scotland after two Russian Bear H aircraft were spotted flying close to UK airspace.
Voyagers based at RAF Brize Norton in Oxfordshire were sent to provide air-to-air refuelling support while communications and radar assistance was given from the National Air Defence Operations Centre.
A Ministry of Defence spokesman said: "RAF Quick Reaction Alert Typhoon fighter aircraft were launched today after Russian aircraft were identified flying close to UK airspace.
"The Russian planes are being escorted by the RAF in the UK area of interest."
The Bears did not enter the UK's sovereign airspace but their appearance will be seen as the second display of Russia's military power near Britain in a matter of hours.
Earlier the Udaloy class destroyer Severomorsk, a tanker and a support ship were monitored by the frigate HMS Argyll as they passed through the Channel while returning from the Mediterranean.
The MoD said no exercises were seen taking place following reports the vessels were set to carry out military drills in the waters.
It follows a flurry of similar incidents in recent months and comes amid strained relations between Moscow and the international community over the crisis in Ukraine.
James Nixey, head of the Russia and Eurasia Programme at Chatham House, said the Channel is a "legitimate shipping lane" but added: "Equally, these things aren't done by accident.
"Russia is trying to show it has got full spectrum capability warfare.
"It is not a prelude to war but it is a reminder that Russia likes to remind us of - that it is a power to be reckoned with, not a fading power, which might be closer to the reality.
"It can tell us that with a degree of braggadocio."
In November the Royal Navy monitored a squadron of Russian warships as they moved through the Strait of Dover after carrying out exercises in the North Sea.
Then in February a Russian warship was tracked as it passed through the English Channel.
On that occasion Yaroslav Mudry and its accompanying tanker, the Kola, were sailing back to Russia after a deployment in the Mediterranean.
British warship HMS Argyll, based in Plymouth, Devon, was deployed and used its Lynx helicopter and sensors to locate and monitor the movement of the Russian ships off the coast of France and through the English Channel.
Today's incidents came as all branches of the British military take part in large-scale military exercises.
The Nato war games, which started on Saturday and run until April 24, include "significant naval and aerial activity" off the west and east coasts of Scotland, the Royal Navy has said.
There is also activity taking place at several other locations in the UK.