Fierce fighting between Ukrainian troops and pro-Russian rebels has broken out in the southeastern city of Mariupol, killing 21 people, Ukraine’s interior minister says.

An attempt by about 60 rebels armed with automatic weapons to storm the police headquarters turned into a “full-scale military clash” when army and interior ministry troop reinforcements arrived, Arsen Avakov said on his official Facebook page.

He said the death toll from the near-two-hour combat stood at 20 rebels and one policeman, while another four policemen were wounded and four rebels were captured.

“A significant part of the terrorist group has been destroyed,” Avakov said.

Mariupol, a port city with a population of 500,000 located close to the Russian border, has been the scene of several clashes between Ukrainian forces and rebels in the past days and weeks – though none as deadly as Friday’s.

Witnesses there said the fighting was ferocious and involved an exchange of automatic gunfire and shelling from eight armoured vehicles.

The police headquarters was gutted by fire and, after the battle, firemen were at the scene trying to extinguish the flames.

The bodies of two men wearing civilian clothes and covered by blankets lay on the road in front of the building.

Near the city hall, which was also blackened by fire, several hastily erected barricades were smouldering and burnt, testament to the sudden and savage fighting.

The violence erupted as Ukraine was observing commemorations for the Soviet victory over German forces in World War II.

It occurred at the same time Russian President Vladimir Putin was making a controversial visit to the annexed peninsula of Crimea to inspect Russian naval forces based there