British ministers will boycott the Paralympics in Sochi over Russia's decision to take military action in Ukraine.
David Cameron said it would be "wrong" for members of the Government to attend the games in Russia because of the "serious situation" in Ukraine.
It comes after William Hague announced the UK has pulled out of preparatory talks due to be held in the coming days for the G8 summit in Sochi.
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The Prime Minister tweeted: "Because of the serious situation in Ukraine, @WilliamJHague & I believe it would be wrong for UK Ministers to attend the Sochi Paralympics."
Britain's attempt to ramp up the diplomatic pressure on Moscow came as hundreds of unidentified gunmen surrounded an infantry base in the region.
The convoy of at least 13 troop vehicles, with Russian number plates, and four armoured vehicles with mounted machine guns, prevented anyone entering or leaving the base in Privolnoye.
Hague said the sovereignty and territorial integrity of Ukraine had been "violated" and called for Moscow to speak directly to the nation's new leaders.
Speaking from RAF Northolt as he was about to board a plane to Kiev, the Foreign Secretary said: "We have to recognise the sovereignty and the territorial integrity of Ukraine has been violated and this cannot be the way to conduct international affairs and so, in addition to calling yesterday's emergency meeting of the UN Security Council, the United Kingdom will join other G8 countries this week in suspending our cooperation under the G8, which Russia chairs this year, including the meetings this week for the preparation of the G8 summit.
"We will keep our approach to further G8 meetings under review."
Ukraine has put its army on full combat alert following the widespread deployment of Russian troops.
Its new prime minister Arseniy Yatsenyuk has called on the Russian premier to pull back his troops, warning "we are on the brink of disaster" while Nato secretary general Anders Fogh Rasmussen urged Moscow to calm the "dangerous" situation.
US secretary of state John Kerry accused Russia of an "incredible act of aggression" and warned that it faces serious sanctions as well as risking its membership of the G8 group of leading nations.
Cameron "remains gravely concerned" about the situation and will talk to US president Barack Obama and premiers in Ukraine's neighbouring countries, Downing Street said.
"The Prime Minister remains gravely concerned about events in Ukraine and is determined to pursue all avenues to reduce tensions and de-escalate a very dangerous situation," a spokesman said.
"This evening he will speak to President Obama, Polish Prime Minister Tusk and Lithuanian President Grybauskaite.
"Earlier today the Foreign Secretary flew to Kiev to meet the new Ukrainian authorities, make clear our support for them, and urge continued restraint.
"In current circumstances, the Prime Minister is clear that the focus of our engagement with Russia must be the situation in Ukraine, rather than other normal business. The Foreign Secretary announced earlier today that the UK will not take part in any of the G8 preparatory meetings in Russia planned for this week."
No 10 said Cameron and the Foreign Secretary decided it would "inappropriate" for ministers to represent the UK at the Sochi games.
"The Prime Minister remains fully supportive of our Paralympic athletes' participation at Sochi," the spokesman added.
"A stable Ukraine is in the interests of everyone - including the UK. Ukraine is one of the largest countries in Europe and a neighbour of the European Union. Its economic prosperity, security and stability matter. The sovereignty and territorial integrity of Ukraine has been violated and this cannot be the way to conduct international affairs."
The Lib Dems were already boycotting the games over Russia's anti-gay laws.
A spokesman for Deputy Prime Minister Nick Clegg said: "Nick Clegg has already made clear that Lib Dem ministers would not be attending the Winter Olympics or the Paralympics in Sochi because of Russia's deeply regressive anti-gay legislation.
"So, Lib Dem ministers would already not have been involved but that position is only strengthened by Russia's deeply concerning response to the situation in Ukraine."
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