NATO's Black Sea members Bulgaria, Romania and Turkey joined four other alliance states in a multinational naval exercise on Tuesday just across the water from Crimea, which was annexed by Russia last year.

The naval rapid reaction force in the Black Sea drills include a U.S. flagship, the guided-missile cruiser USS Vicksburg and ships from the six other participating states. Canada, Germany and Italy are also taking part.

NATO has held a series of exercises in Eastern Europe since Moscow's move into the region to reassure members jittery about Russian intentions in the wake of the Ukraine crisis.

The training will include simulated anti-air and anti-submarine warfare exercises, as well as simulated small boat attacks and basic ship handling maneuvers, a Bulgarian navy spokesman told Reuters.

Last week, the Russian Defense Ministry said its forces had begun large-scale military exercises in southern Russia and in the disputed territories on Russia's borders, including the Crimean region.

Relations between Russia and the West are at their most strained since the Cold War. Ukraine and the West accuse Moscow of directing a separatist assault in eastern Ukraine with its own troops and weapons. Russia has denied those accusations.

Once a satellite state of the Soviet Union, Bulgaria joined NATO in 2004 and is one of six Eastern European countries that will host new command units staffed with national and NATO soldiers being set up in response to the Ukrainian crisis.