U.S. Vice President Joe Biden on Tuesday blasted Russia's annexation of Crimea, saying borders should not be changed at gunpoint, as he began a visit to Romania days before a crucial vote in Ukraine.
"What Russia has done violates not just Ukraine's sovereignty, but a fundamental principle we fought for in the 20th century and thought we had clearly established," Biden told Romanian and US troops at the Otopeni military airbase near Bucharest.
"Europe's borders should never again be changed at the point of a gun, which is why we continue to condemn Russia's illegal occupation of Crimea."
Ukraine is due to hold a presidential vote on Sunday as it struggles to overcome a political crisis that threatens its very existence after Russia annexed Crimea and pro-Moscow rebels took up arms in the east against Kiev.
The US Vice President warned that the West would "impose greater costs on Russia" unless Moscow ceased efforts to destabilize Ukraine.
Biden is expected to lobby hard for support from Cyprus for fresh sanctions against Moscow when he heads to Nicosia on the second leg of his European trip.
Cyprus has stood firmly against such measures due to fears sanctions directly targeting the Russian economy could hurt its own fortunes as its finance industry is a key conduit of Russian investment and savings.
Biden also sought to reassure Romania's leaders that the former Communist state will be protected by NATO in the event of Russian aggression.
Addressing a crowd of airmen, the vice president praised Bucharest for being a "devoted NATO ally" and paid tribute to the 26 Romanian soldiers who gave their lives in Iraq and Afghanistan.
Romania, as well as Poland and the Baltic States, has become increasingly worried by Russia's actions in the region and has repeatedly called for a stronger NATO presence in Eastern Europe.
"Our strategy is about more than just imposing cost. It's fundamentally about investing in a revitalized NATO that emerges from this crisis... stronger and more united," he said.
"Aggression in Crimea, less than 250 miles from Romanian territory, from NATO's borders, reminds us why we need NATO."
Biden said the US's commitment to act under article five was "a sacred obligation, not just for now but for all time".
Article five of NATO's Washington Treaty states that an attack on one member is an attack on all and requires a military response by all.
"I am here to say on behalf of the president: you can count on us," Biden told Romanian soldiers and officials.
The U.S. and NATO allies have "stepped up... military presence in the air, land and on the sea of NATO's eastern flank," he added.
A new U.S. missile cruiser, the Vella Gulf, is due to arrive shortly in the Black Sea.