US Vice President Joe Biden on Wednesday said Washington could send forces to the Baltic states to reassure ex-Soviet NATO allies rattled by Russia's takeover of Crimea.
"We are exploring a number of additional steps to increase the pace and scope of our military cooperation, including rotating US forces in the Baltic region to conduct ground and naval exercises and training missions," Biden said during a visit to Lithuania, after a first stop in Poland.
Russia's swift absorption of Crimea has alarmed the Baltic states, which were ruled by the Kremlin for five decades after World War II.
"Under Article 5 of the NATO Treaty, we will respond, we will respond to any aggression against a NATO ally," Biden said, spelling out Washington's commitment to protecting its allies under NATO's mutual defence clause.
Lithuanian President Dalia Grybauskaite accused Russia of using "brutal force to redraw the map of Europe" over Moscow's move to absorb Ukraine's strategic peninsula, which is home to Russia's Black Sea fleet.
"The situation is a direct threat to our regional security," she said at the joint press conference.
Biden repeated an earlier warning to Moscow that it faces increasing isolation, after Ukraine said its Crimea bases had been attacked by Russian forces.
"As long as Russia continues on its dark path, they will face increasing political and economic isolation," Biden told reporters in Vilnius.
Estonia, Latvia and Lithuania joined the EU and the NATO Western military alliance in 2004 after breaking free from the crumbling Soviet Union in 1990-91.
Relations between the trio and Moscow have remained tense ever since.
Washington has already sent six extra F-15 fighter jets to step up NATO's air patrols over the Baltics, and Britain on Monday offered its Typhoon aircrafts.