(Adds Biden quotes, detail, background)
* Ukraine tops agenda as US VP meets Romanian leaders
* Biden to head to Nicosia after Bucharest
* Romania a staunch backer of sanctions against Moscow
By Luiza Ilie
BUCHAREST, May 20 (Reuters) - U.S. Vice President Joe Biden arrived in Bucharest on Tuesday as part of Washington's campaign to reassure east European allies of American backing in the West's confrontation with Russia over Ukraine.
A former communist state on the Black Sea, Romania joined NATO a decade ago and the European Union in 2007. It has been among the most vehement advocates of Western sanctions against Moscow after Russia annexed Crimea from Ukraine.
"You can count on us, we do what we say," Biden told a gathering of Romanian and American troops after doffing his brown leather jacket at a military air base near Bucharest, where "capacity-building" exercises are being conducted.
Biden's White House office had earlier said he would underscore Washington's "ironclad commitment to the collective defense of NATO under Article 5 and express appreciation for Romania's contributions to regional and global security".
Delivering his speech inside a brightly-lit hangar, flanked by Romanian military aircraft, Biden said: "The aggression to Crimea ... reminds us why we need NATO. Europe's borders should never again be changed at the point of a gun ... which is why we continue to condemn Russia's occupation of Crimea."
Romania, which neighbours Ukraine, has said NATO must reposition its resources in the wake of Moscow's manoeuvres. It is especially wary that Moldova - a tiny state bordering Romania with a Russian-speaking minority - could be next in Moscow's sights given the risk of separatist unrest emerging there.
Biden is expected to hold talks with President Traian Bsescu and Prime Minister Victor Ponta on Wednesday and address the need to deepen economic ties, recommit to strengthening democratic institutions and undertake sustained efforts to bolster Europe's energy security.
Russia's annexation of Crimea and its support for separatist movements in the east and south of Ukraine have drawn economic sanctions from the United States and the European Union.
Since the standoff between Russia and the West began, Romania has taken part in navy drills in the Black Sea and hosted military exercises with U.S. troops.
Biden's journey Tuesday through Friday, which will include a stopover in Cyprus, is part of a campaign to underline U.S. solidarity with allies during the Ukraine crisis that has included trips to Kiev, the Baltic states and Poland. (Writing by Radu Marinas; Editing by Mark Heinrich)