A search and rescue operation was under way on 7 June after a border patrol boat with a crew of seven sank off the port city of Mariupol in Ukraine following an explosion, border guard authorities said.

"Alarm systems went off on many cars, so the boom was quite loud. I did not see any flames, but a black smoke, I did not see anything else," local resident Irina, who was on a beach when the explosion happened, told Reuters.

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.

But the border guard authorities said the cause of the explosion that caused the boat to go down in the Sea of Azov was not immediately clear.

According to the state border guard statement, two border guard staff were injured and the boat suffered substantial damage.