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.
Suspected Russian troops have surrounded a Ukrainian military base
It came amid warnings the deepening crisis is just "a pace away from catastrophe", where the smallest act could take it "over the edge".
The US Secretary of State John Kerry has 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.
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.
Ukrainian soldiers have blocked the entrance to the base with a tank
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.
UK Foreign Secretary William Hague, who is flying to the Ukrainian capital Kiev, said the country's sovereignty and territorial integrity had been "violated".
He said: "Our response is diplomatic and peaceful and it should be. That is our response but it will be a very united diplomatic response and not just from the Western world, I think, but from many other countries in the world and that is something that Russia will have to think hard about."
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.
People attend a rally in Kiev against Russian intervention in Ukraine
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, Ukraine withdrew its coastguard vessels from two ports in Crimea and moved them to other Black Sea bases, in a sign that Russia was completing its seizure of the peninsula.
However, Ukraine claims it still has a fleet of 10 ships in the Crimean port of Sevastopol, which remain loyal to Kiev.
There have also been more 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.
People pose for a picture in front of an armoured vehicle in Crimea
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."
"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."
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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