The residents of Prymorskiy and Zhovtneviy districts of Mariupol reported hearing a huge explosion blast coming from the direction of the sea at about 1200, according to the local news agency 0629.
The force of the blast shook window panes, the agency reported.
"The explosion occurred in the Azov sea outside the 20-mile zone, within territory controlled by the Russian Federation," according to press secretary of the Anti-Terrorist Operation headquarters in Mariupol Dmytro Horbunov.
No border guards or Ukrainian Navy officers were injured in the blast, he said. The cause of the explosion is being investigated.