Ukraine's Prime Minister Arseniy Yatsenyuk has warned his crisis-hit country was on the "brink of disaster", accusing Russia of declaring war on his country.
"If President Putin wants to be the president who started a war between two neighbouring and friendly countries, between Ukraine and Russia, he has reached his target within a few inches. We are on the brink of the disaster," he told reporters in English on Sunday, a day after Russia's parliament approved use of military force on Ukraine.
His concerns have been echoed by United States Secretary of State John Kerry, who has condemned Russia's "incredible act of aggression" in Ukraine and threatened "very serious repercussions" from the US and other countries including sanctions to isolate Russia economically.
"You just do not, in the 21st century, behave in 19th century fashion by invading another country on completely trumped up pre-text," Kerry said on American broadcaster CBS on Sunday.
"It's an incredible act of aggression. It is really a stunning, willful choice by President [Vladimir] Putin to invade another country. Russia is in violation of the sovereignty of Ukraine. Russia is in violation of its international obligations," Kerry said, adding that Russia still has "a right set of choices" that can be made to defuse the crisis.
The West is facing its biggest confrontation with Moscow since the Cold War after Putin declared the right to use force in its neighbour and Ukraine put its military on alert.
Unidentified gunmen believed to be Russian have gained the control of the state buildings, including airports, in Crimea, an Black Sea peninsula where ethnic Russians are the majority and Moscow has a naval base.
In his remarks, Kerry said US President Barack Obama told Putin in a 90-minute phone call on Saturday that "there will be serious repercussions if this stands. The president... told Mr. Putin that it was imperative to find a different path, to roll back this invasion and undo this act of invasion."
Mention of sanctions
He also said that Moscow risked losing its place among the prestigious Group of 8 developed nations and end up in isolation over its deployment of troops in its neighbour.
"They're [G8 countries] prepared to put sanctions in place, they're prepared to isolate Russia economically, the ruble is already going down. Russia has major economic challenges," Kerry said, as he also mentioned visa bans, asset freezes and trade isolation as possible steps.
Meanwhile, NATO's secretary-general warned Moscow it was "threatening peace and security in Europe" with its seizure of Crimea, and should "de-escalate tensions".
Speaking moments before chairing an emergency meeting of NATO ambassadors in Brussels, Anders Fogh Rasmussen said, "What Russia is doing now in Ukraine violates the principles of the United Nations charter."
The statements came amid a chorus of condemnation from Washington and its allies. Kerry made his remarks on Russia's G8 membership only hours after the US, Canada, France and the United Kingdom have pulled out of the talks in preparation for a G8 meeting in Sochi scheduled for the summer of 2014.