Pentagon chief Chuck Hagel toured a warship stationed at a Romanian Black Sea port Thursday, reaffirming the U.S. commitment to the security of allies worried over Russia's intervention in Ukraine.

"The presence of the USS Vella Gulf is a clear expression of this commitment" to the collective security of NATO allies, Hagel told journalists at the port of Constanta.

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.

"Europe's borders should not be changed at the point of a gun," he told Romanian and U.S. troops at the Otopeni military airbase near Bucharest.

Vice President Joe Biden will underscore a firm U.S. commitment to defend NATO partners and rally support for tougher potential sanctions against Russia on a trip to Romania and Cyprus this week.

Biden will arrive in Bucharest on Tuesday at what one senior official called a "complicated and challenging time in Europe" fostered by Russia's "destabilizing" actions in Ukraine.

The number of Romanians and Bulgarians employed in Britain has dropped since the start of the year despite the lifting of entry restrictions, official figures showed Wednesday.

Politicians and many newspapers had expressed fears of a wave of immigrants from the two poorest countries in the European Union once the restrictions ended on January 1.

Romanian President Traian Basescu toured a U.S. warship stationed at the Black Sea port of Constanta on Monday, thanking the country's NATO allies for their "solidarity" in the face of Russia's actions in neighboring Ukraine.

Basescu visited the guided missile destroyer USS Donald Cook, stationed in Constanta along with French reconnaissance ship Dupuy de Lome after the eruption of the Ukrainian crisis.

NATO should redeploy its forces in eastern Europe and make the Black Sea region a priority after Russia's annexation of Crimea, Romania's foreign minister urged in an interview Thursday with Agence France Presse.

"Romania has concrete expectations of a redeployment and an eastward repositioning of NATO's naval, air and ground forces," Titus Corlatean said.

Romania has approved an increase in American troops at its military airbase on the Black Sea as Washington continues to shift its main transit base for Afghanistan away from Kyrgyzstan, a report said Tuesday.

Romanian President Traian Basescu sent a letter to parliament approving the U.S. request for 600 more troops at the Mihail Kogalniceanu airport in the southeast of the country, the Agerpres news agency said.

Romanian President Traian Basescu on Wednesday warned the former communist state could become a target for spies in the wake of Moscow's annexation of Crimea.

"We must definitely take into account the risks of growing espionage activities on Romania's territory in the wake of developments in the region," Basescu said.

Romania's president said Monday NATO should reposition its forces after Russia's annexation of Crimea.

Traian Basescu said he would discuss the crisis in Ukraine, which shares a border with Romania, with world leaders meeting in The Hague.

The United States on Friday officially opened a new military transit hub in Romania which will play a key role for American forces leaving Afghanistan.

Major General John O'Connor, in charge of logistical support, said the Mihail Kogalniceanu airbase near the Black Sea would act as a "center of gravity" for the U.S. draw down.