KIEV, March 19 - The United States warned Moscow it was on a "dark path" to isolation on Wednesday as Russian troops seized two Ukrainian naval bases, including a headquarters in the Crimean port of Sevastopol where they 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 U.S. Vice President Joe Biden, referring to reports of armed attacks against Ukrainian military personnel in Crimea, Reuters reported.

Biden was in the Lithuanian capital, Vilnius, as 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 postwar architecture of Europe," Lithuanian President Dalia Grybauskaite said.

The head of NATO warned that Putin may not stop with the annexation of Crimea and urged Europe to step up defense spending in response to the crisis.

"Crimea is one example. But I see Crimea as an element in a greater pattern, in a more long-term Russian, or at least Putin, strategy," Secretary-General Anders Fogh Rasmussen told a Washington think tank. "So of course our major concern now is whether he will go beyond Crimea.”

U.N. 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 neutralize any threat of armed resistance in Crimea.

"This morning they stormed the compound. They cut the gates open, but I heard no shooting," said Oleksandr 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.

Russian troops seized another Ukrainian naval facility in Crimea late on Wednesday.

"Russian troops came and asked us to leave the base, which we did," Ukrainian navy Major Eduard Kusnarenko told Reuters outside the base in Bakhchisaray, about 30 km (20 miles) southwest of the regional capital, Simferopol.

Ukrainian security chief Andriy Parubiy said the Kiev government would urge the United Nations to declare Crimea a demilitarized zone.

"The Ukrainian government will immediately appeal to the United Nations to recognize Crimea as a demilitarized zone and take necessary measures for Russian forces to leave Crimea and prepare conditions for redeployment of Ukrainian forces," Parubiy said.

Ukraine announced plans to introduce visas for Russians, and Russia said it might respond in kind.

Putin said his move to annex Crimea was justified by "fascists" in Kiev who overthrew pro-Moscow President Viktor Yanukovych last month. (rt/ez)