Ukraine's Prime Minister Arseniy Yatsenyuk has warned his crisis-hit country is on the "brink of disaster", accusing Russia of declaring war on his nation.
"If President [Vladimir] 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 the use of military force in Ukraine.
Ukraine's navy chief said on Sunday that he had switched allegiance to Crimea's pro-Russia authorities.
Kiev said it had launched a treason case against Denis Berezovsky, who surrendered his headquarters in the port of Sevastopol on only his second day on the job.
Yatsenyuk's concerns were echoed by US Secretary of State John Kerry, who 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 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.
In his remarks, Kerry said US President Barack Obama had 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."
Kerry said he would fly to Kiev late on Monday for discussions there on Tuesday.
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 Russians, have gained control of state buildings and airports, in Crimea, a Black Sea peninsula where ethnic Russians are the majority and Moscow has a naval base.
Kerry also said that Moscow risked losing its place among the prestigious Group of 8 (G8) developed nations and could end isolated over its deployment of troops in its neighbour.
|US weighs up sanctions on Russia over Ukraine
"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.
On Sunday, the other countries in the G8, Canada, France, Germany, Italy, Japan, the UK and the US issued a statement condeming Russia's actions.
"We note that Russia’s actions in Ukraine also contravene the principles and values on which the G7 and the G8 operate," the statement said.
"As such, we have decided for the time being to suspend our participation in activities associated with the preparation of the scheduled G-8 Summit Sochi in June, until the environment comes back where the G8 is able to have meaningful discussion.
Meanwhile, NATO's secretary-general warned Moscow that it was "threatening peace and security in Europe" with its seizure of Crimea, and should "de-escalate tensions".
Speaking after chairing an emergency meeting of NATO ambassadors in Brussels, Anders Fogh Rasmussen said that Russia was violating the principles of the UN charter and the international law.
"NATO’s allies will continue to support Ukraine’s sovereignty and territorial integrity," he said.
"We urge both parties to immediately seek a peaceful solution through bilateral dialogue, with international facilitation ... and through the dispatch of international observers under the auspices of the United Nations Security Council or the Organisation for Security and Cooperation in Europe," he said.