The US Navy has reportedly begun military manoeuvres along with Romanian and Bulgarian navies near Russia's borders, fuelling tensions in the region.
The joint military training exercises are said to represent the US's resolve to protect its NATO allies, Reuters reports.
USS Truxtun, the US Navy's Arleigh Burke-class guided-missile destroyer with a crew of about 300 will take part in the drills, as well as Bulgarian naval frigate Drazki and three Romanian naval vessels, Bulgaria's defence ministry said in a statement.
The training exercise will not involve any live-firing and will be held off the Black Sea, across the water from the Crimean Peninsula, where Russian military groups have seized control from Ukrainian authorities.
"The goal of the exercise is to increase the operational compatibility within NATO members' navy vessels, to improve the mutual understanding and to increase the crews' preparedness at sea," the ministry said.
Meanwhile, US military fighter jets are scheduled to take part in joint air defence exercises in north Poland, with Poland's president set to review the manoeuvres.
The US said that the drills were planned before the Ukraine crisis, although both exercises are alleged support gestures for NATO members after Russia's intervention in Ukraine.