UK & World News
Ukraine Mobilises Troops As Crisis Deepens
Ukraine's leader has placed its military on full combat alert and called up all its reserves after Russia gave the go-ahead to send more troops into the country.
It came as it was warned the volatile crisis is just "a pace away from catastrophe", where one foolish act could take it "over the edge".
Russian President Vladimir Putin has warned the United States that he retains the right to protect Russia's interests in east Ukraine and Crimea as tensions escalate.
Mr Putin issued the stark warning in a 90-minute phone call with US President Barack Obama, in which the two leaders disagreed over the deepening crisis.
A statement from the Kremlin said unrest in Ukraine posted "a real threat to the lives and health of Russian citizens and the many compatriots who are currently on Ukrainian territory".
"Russia retains the right to protect its interests and the Russian-speaking population of those areas," the statement added.
During the call, Mr Obama accused Russia of a "clear violation of Ukrainian sovereignty and territorial integrity", and expressed his "deep concern" over Russia's actions.
The White House said: "The United States calls on Russia to de-escalate tensions by withdrawing its forces back to bases in Crimea and to refrain from any interference elsewhere in Ukraine."
It came as former Liberal Democrat leader Paddy Ashdown, himself a former special forces officer, told Sky's Murnaghan programme: "I think we are a pace away from catastrophe at the moment.
"It would require one foolish act, I don't know, a trigger happy Russian soldier, a Ukrainian guard who acts aggressively at one of these institutions that has been taken over by Russia or Russian supporters.
"A foolish act now could tip us over the edge.
"The one thing that is absolutely essential now is that the West speaks with a single voice.
"The smallest tremor, the smallest act now could take us over the edge."
This was echoed by Sir Tony Brenton, the former British ambassador to Russia, who told Murnaghan: "It only requires one person to make a mistake for things to go very badly wrong."
Former Foreign Secretary Sir Malcolm Rifkind also warned of the dangers posed by the deepening crisis.
He told Dermot Murnaghan: "There is no doubt this is probably the most serious crisis since the Cold War.
"This has to be a defining moment in the West's relationship with Russia.
"There are very serious implications for the whole of Europe."
The Kremlin has been accused of sending 6,000 troops into Crimea, despite calls from the US and Britain for Moscow to back off.
Two Russian anti-submarine warships have also appeared off the Crimea coast, violating an agreement on Moscow's lease of a naval base, Interfax news agency reported.
The two vessels, part of Russia's Baltic Fleet, have reportedly been sighted in a bay at Sevastopol, where Moscow's Black Sea Fleet has a base.
Sky's Dominic Waghorn said Mr Obama and Mr Putin appear to be in disagreement over what should be done to de-escalate the crisis.
"The phone call didn't go very well, it seems. We have each leader, it looks like, lecturing each other," he said.
"The problem clearly here is that Barack Obama may regard Vladimir Putin as a throwback to the Cold War who has ended up somehow running Russia in 2014.
"The two leaders have no real way, on a personal level at least, of working it out and resolving this crisis."
In response to Russia's aggression, the US has already announced it will suspend participation in preparatory meetings for the G8 summit in Sochi in June.
European foreign ministers are also due to hold emergency talks in Brussels on Monday.
British Foreign Secretary William Hague said he was "deeply concerned" by the situation and the decision of the Russian parliament to authorise military action.
"This action is a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine. We condemn any act of aggression against Ukraine," he said.
Mr Hague is due to visit the country today and hold talks with Ukraine's new leaders.
Pro-Russia demonstrations have been held in major cities in the east and south of Ukraine, where supporters of the new Ukrainian government in Kiev have been beaten up.
The autonomous republic of Crimea has a prime minister loyal to Moscow, but the government in Kiev has vowed to maintain the country's territorial integrity.
Reports suggest that Russian troops are trying to occupy an anti-aircraft missile base and have taken control of airports on the peninsular.
Ukraine's border guard service said about 300 armed men were attempting to seize its main headquarters in the port city of Sevastopol.
Russian helicopter gunships have also been seen in Ukrainian airspace.
But there were claims by Russia that gunmen from Kiev had been sent to seize the offices of Crimea's interior ministry.
There was grainy footage of an unidentified armed group breaking into a building, with smoke rising and reports that people had been "wounded".
In what appears to be an orchestrated move, a referendum on whether residents in Crimea want greater independence from Ukraine has been brought forward by two months to March 30.
Russia's Lower House of Parliament, the Duma, has called on Mr Putin†"to take measures to stabilise the situation in Crimea".
Ukraine's population is divided in loyalties between Russia and the West.
Much of western Ukraine advocating closer ties with the European Union. However, the eastern and southern regions look to Russia for support.
Crimea has 2.3 million inhabitants, most of whom identify themselves as ethnic Russians and speak Russian.
:: Watch Sky News live on television, on Sky channel 501, Virgin Media channel 602, Freeview channel 82 and Freesat channel 202.