A search operation was underway on Sunday for a missing border guard official after a patrol boat sank off Ukraine’s port city of Mariupol following an explosion, border guard authorities said.

Six crew members were injured.

The cause of the explosion was not immediately clear, authorities said.

Mariupol is on the edge of the conflict between government forces and Russian-backed separatists and has been under threat from rebel attacks for months.

“Today at 14.20 hrs a patrol boat exploded as it came out of its mooring station in Mariupol,” a state border guard statement said. It went down in the Sea of Azov.

“There were seven service staff on board. Six of them received injuries of various degrees. The search is continuing for one service staff member,” it said.

The boat suffered substantial damage, it said.

Reuters