The story comes from The Telegraph.
Ukraine live: Britain pulls out of pre-G8 talks in Sochi as Ukraine 'on the brink'
Britain has pulled out of G8 planning talks, due to be held in Sochi, as the interim prime minister of Ukraine says his country is "on the brink of war" after Russia approved the use of force. All the latest news from Ukraine here
13.33 Ukraine's acting foreign minister has apparently told the BBC he's been trying for three days to speak to Sergei Lavrov, his Russian counterpart - but hasn't been able to.
Incredible if true.
13.18 Britain has pulled out of preparatory talks due to be held in the coming days for the G8 summit in Sochi over Russia's decision to take military action in Ukraine, Foreign Office officials have said.
13.15 Ukraine's defence ministry has confirmed the story that Roland Oliphant brought you at 11.35 - the surrounding of the base.
"One thousand armed fighters and around 20 trucks are blocking the perimetre of the 36th brigade of border guards... in Perevalne," the ministry said in a statement.
13.11 Pope Francis has been giving his usual Sunday address to thousands of people in St. Peter's Square.
He urged all segments of Ukrainian society to work together to overcome their misunderstandings and build a future together.
13.00 It's now 8am in New York: America is waking up and digesting the news from the past 12 hours.
Here is a good US-focused round up of the events, from the LA Times.
In the days ahead, Rep. Eric Cantor (R-Va.), the majority leader, said "Congress will be looking for ways to help bolster the Ukrainian government's efforts to stabilize the economy, restore their sovereignty, and promote political reconciliation."
12.43 In other news... A bagpiper is playing in Kiev's Independence Square. Not sure why.
12.33 Lithuania has recalled its ambassador from Russia for consultations.
In a brief statement, the Baltic nation's foreign ministry said the move was in reaction to "illegal Russian actions against Ukraine's sovereignty and territorial integrity".
Canada last night recalled its ambassador to Moscow, too.
Britain has not recalled its ambassador - but last night summoned the Russian ambassador in London to the Foreign Office.
The Czech Republic this morning did the same in Prague.
Russia's upper house also demanded the recall of Russia's ambassador to the US - something it has not yet done.
12.21 Amid the rising diplomatic temperature, it's worth re-reading Roland Oliphant's soothing dispatch from Crimea yesterday.
12.17 GERMAN FOREIGN MINISTER SAYS ESCALATION OF TENSIONS IN UKRAINE IS "EXTREMELY DANGEROUS"
12.13 Last night Russian warships were spotted off the coast of Crimea, in violation with Russia's agreement with Ukraine over the Sebastapol base.
Russia has controlled that base, remember, for 230 years.
They are allowed to keep their Black Sea fleet there but cannot just do as they wish - they agreed with Ukraine to keep Kiev informed.
Russian forces also took control of the Coast Guard offices in Balaklava - again violating the agreement.
12.10 Anders Fogh Rasmussen, the Nato secretary general, has convened a meeting of Nato.
He's just given this statement, ahead of the meeting, which accuses Russia of violating the principles of the UN Charter:
Quote I have convened the North Atlantic Council today because of Russia’s military action in Ukraine. And because of President Putin’s threats against this sovereign nation.
What Russia is doing now in Ukraine violates the principles of the United Nations Charter. It threatens peace and security in Europe. Russia must stop its military activities and its threats.
Today we will discuss their implications, for European peace and security, and for NATOs’ relationship with Russia.
Afterwards, we will meet in the NATO-Ukraine Commission.
We support Ukraine’s territorial integrity and sovereignty. We support the right of the people of Ukraine to determine their own future without outside interference. And we emphasise the need for Ukraine to continue to uphold the democratic rights of all people and ensure that minority rights are protected.
Ukraine is our neighbour, and Ukraine is a valued partner for NATO.
We urge all parties to urgently continue all efforts to move away from this dangerous situation. In particular, I call on Russia to de-escalate tensions.
11.57 In Crimea, Roland Oliphant has managed to get a few words with the Ukrainian commander of the base which is being surrounded by Russians (see 11.35).