The United States warned Moscow it was on a "dark path" to isolation on Wednesday after Russian troops stormed Ukraine's naval headquarters in the Crimean port of Sevastopol and raised their flag.
The dramatic seizure came as Russia and the West dug in for a long confrontation over Moscow's annexation of Crimea, with the United States and Europe groping for ways to increase pressure on a defiant Russian President Vladimir Putin.
"As long as Russia continues on this dark path, they will face increasing political and economic isolation," said US Vice President Joe Biden, referring to reports of armed attacks against Ukrainian military personnel in Crimea.
Read: All you need to know about Ukraine
Biden was in the Lithuanian capital Vilnius, part of a quick trip to reassure Baltic allies worried about what an emboldened Russia might mean for their nations.
Lithuania, along with Estonia and Latvia, are NATO members.
"There is an attempt, using brutal force, to redraw borders of the European states and to destroy the post-war architecture of Europe," Lithuanian President Dalia Grybauskaite said.
United Nations chief Ban Ki-moon meets Putin in Moscow on Thursday and travels to Kiev on Friday. He will urge a peaceful end to a crisis that began when Ukraine's president abandoned a trade pact with the European Union and turned instead to Moscow, prompting violent street protests that led to his overthrow.
Russian lawmakers raced to ratify a treaty making Crimea part of Russia by the end of the week, despite threats of further sanctions from Washington and Brussels.
The Russian military moved swiftly to neutralise any threat of armed resistance in Crimea.
Read: Obama: Russia has violated international law in Ukraine
"This morning they stormed the compound. They cut the gates open, but I heard no shooting," said Oleksander Balanyuk, a captain in the navy, walking out of the compound in his uniform and carrying his belongings.
"This thing should have been solved politically. Now all I can do is stand here at the gate. There is nothing else I can do," he told Reuters, appearing ashamed and downcast.
Ukrainian military spokesman Vladislav Seleznyov said the commander of the Ukrainian navy, Admiral Serhiy Haiduk, was driven away by what appeared to be Russian special forces.
In Washington, the White House condemned Russian moves to seize Ukrainian military installations, saying they are creating a dangerous situation. NATO accused Russia of trying to "redraw the map of Europe".
Read: Putin has chosen path to Russia's isolation: Britain