Ukraine has launched a treason case against its new navy chief after he switched allegiance to the pro-Russian Crimea region amid escalating tensions in the country.
Rear Admiral Denis Berezovsky was appointed head of Ukraine's navy on Saturday and the Kiev government was claiming that its Black Sea fleet remained loyal just a few hours ago.
Ukraine's leader has accused Russia of declaring war on his country and warned the nation was on the "brink of disaster".
Appealing to the international community for help, Prime Minister Arseniy Yatsenyuk said: "This is the red alert, this is not a threat, this is actually a declaration of war to my country."
Ukraine has mobilised its military and called up all its reserves after Russia gave the go-ahead to send more troops into the country, in what has become the biggest confrontation between Moscow and the West since the Cold War.
Observers say the deepening crisis is just "a pace away from catastrophe", where the smallest act could take it "over the edge".
US President Barack Obama spoke to British Prime Minister David Cameron about the crisis on Sunday night.
They agreed that Russia's actions were "completely unacceptable" and said there must be "significant costs" if Moscow does not change course.
The US Secretary of State John Kerry condemned Moscow's "incredible act of aggression" in Ukraine, and warned of "very serious repercussions" including sanctions to isolate Russia economically.
"You just don't in the 21st century behave in a 19th century fashion by invading another country on a completely trumped up pretext," he told the CBS programme Face The Nation.
US officials said Mr Kerry will visit Kiev on Tuesday to lend support to the new interim leaders, and said Russian forces were "now in complete operational control of the Crimean peninsula."
Nato's Secretary General Anders Fogh Rasmussen has called on Russia to de-escalate tensions.
"What Russia is doing now in Ukraine violates the principles of the United Nations charter. It threatens peace and security in Europe," he said.
In response to the military action in Ukraine, Britain and France have pulled out of preparations for a summit of world leaders in the Russian resort of Sochi in June.
British Prime Minister David Cameron also said it would be "wrong" for ministers to attend the Paralympics in Sochi because of the "serious situation" in Ukraine.
UK Foreign Secretary William Hague, who has flown to the Ukrainian capital Kiev, said the country's sovereignty and territorial integrity had been "violated".
Russian forces have taken over the Ukraine's southeast Crimea region where Moscow has a naval base, and more troops are on their way, prompting accusations of a "military invasion".
Despite mounting international condemnation, Russian President Vladimir Putin has declared he has the right to protect Russian citizens and interests in Ukraine.
This is the same justification used in the 2008 invasion of Georgia over two breakaway regions, which have large ethnic Russian populations.
On Saturday, US President Barack Obama warned Mr Putin during a 90-minute phone call that Russia had flouted international law and urged him to withdraw forces.
In the latest development of the unfolding crisis, hundreds of suspected Russian troops have surrounded a Ukraine military base, preventing soldiers from going in or out.
The convoy blockading the site near the region's capital Simferopol includes at least 17 military vehicles, which have Russian number plates.
The Ukrainian personnel inside have blocked the gate with a tank.
Meanwhile, there have been pro-Moscow demonstrations in east Ukraine, where most people speak Russian.
But in Kiev's Independence Square, where months of protests led to the downfall of pro-Russian President Viktor Yanukovych, there were demonstrations against military action.
Speaking about the Ukraine, former Liberal Democrat leader Paddy Ashdown, who served in the special forces, 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."
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."
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.
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