Ukraine's border guards service said one of its cutters exploded while on patrol Sunday off the shore of the strategic port city of Mariupol.

Six crewmen were injured and rescuers were searching for a seventh.

The regional police department said the boat hit an explosive device, but did not give further details.

Fighting between pro-Russian separatists and government forces near Mariupol has persisted for months, despite a cease-fire called in February and the subsequent reduction in overall hostilities in the east.

Mariupol is the major coastal city between mainland Russia and the Russia-annexed Crimean peninsula. Rebel seizure of the city could help establish a land bridge corridor connecting the two.