Vice President Joe Biden warned Russia it was headed down a “dark path” to isolation and vowed that the U.S. would defend its Baltic allies from Moscow's “aggression.”

Biden, on a three-day trip to Eastern Europe to reassure nations there, spoke from Vilnius, the capital of Lithuania, where he met with that country's leader and the head of Latvia.

“We stand resolutely with our Baltic allies in support of the Ukrainian people and against Russian aggression,” said Biden. “As long as Russia proceeds along this dark path, they will receive increasing political and economic isolation."

Biden also referenced news reports of “armed attacks against Ukrainian military personnel” as troubling.

“Russia cannot escape that the world is changing and rejecting outright their behavior,” Biden continued. “And that there is a price to pay for naked aggression."

Biden's comments come a day after Russian President Vladimir Putin brushed off the West's warnings and signed a treaty annexing Ukraine's Crimean peninsula into Russia.

Russian military forces seized control of the region after Moscow-backed former Ukrainian President Viktor Yanukovych was removed from power. On Sunday the region voted in a referendum to secede from Ukraine and join Russia.

Moscow says that it is protecting the safety of the area's Russian-ethnic majority, but the U.S. and other nations have slammed the move as a violation of international law.

President Obama has said the U.S. will not accept the referendum and has vowed further “costs” to Russia's action. The U.S. and the European Union have targeted a number of Russian and Ukrainian officials for sanctions, but many critics said those measures were ineffective and would not force Putin to back off.

Biden also told leaders that the administration backed Europe's efforts to “diversify your energy sources” and said those steps were important to prevent energy from being used as a “quasi-military weapon in the future.”

Efforts to put tough sanctions on Russia have been hampered by Europe’s dependence on Moscow for energy.