Bulgaria was hosting joint navy drills with fellow NATO members in the Black Sea on Friday, as the United States tries to reassure a region worried about Russia's intervention in Ukraine.

Navy ships from the United States as well as Greece, Romania and Turkey, and four destroyers from NATO's maritime immediate reaction force and a US patrol aircraft, were to take part in the exercises, Bulgaria's defence ministry said.

The drills will continue until July 13.

The manoeuvres are taking place in Bulgarian waters around 250 kilometres (150 miles) from the Crimean peninsula, which was controversially annexed by Moscow in March.

Since then, the United States has regularly deployed troops to eastern European countries for joint drills and sent ships to the Black Sea for exercises with NATO states in the area.

The newest exercises will include crisis response training, the Bulgarian defence ministry said, without elaborating.

Bulgaria joined NATO in 2004.