Pentagon chief Chuck Hagel toured a warship stationed at a Romanian Black Sea port Thursday, reaffirming the US 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.

USS Vella Gulf, a guided missile cruiser, is the fourth warship deployed by the US in the Black Sea since the Ukrainian crisis erupted in March.

President Barack Obama on Tuesday unveiled a $1 billion US security plan for eastern Europe aimed at allaying fears over a resurgent Kremlin and the pro-Russian uprising in ex-Soviet Ukraine.

The plan will include more US troop rotations for exercises and training, and a stronger presence of US ships in the Black Sea, Hagel stressed.

"We will sustain this tempo going forward," the US defence secretary said.

"We are also stepping up our cooperation with other partners and allies surrounding the Black Sea -- including Bulgaria, Georgia, Turkey, and Ukraine."

Hagel reiterated his call to NATO members to boost their defence spending.

"This week in Brussels, I emphasised that, as the US increases its commitments to European security, we are looking for our European allies to do the same," he said.

Defence budgets have fallen in Europe in recent years, but several east European countries including Poland, Romania, the Baltic states and the Czech Republic have announced plans to reverse the trend.