Ukrainian forces have regained control of the port of Mariupol, raising the national flag over the southeastern city's main administrative building, Interior Minister Arseny Avakov said this morning.

"At 10.34am (8.34am Irish time) the Ukrainian flag was raised over City Hall in Mariupol," he wrote on Facebook.

Ukrainian forces attacked the city as part of a broader military operation to reclaim territory seized by pro-Russian separatists.   

There was no immediate comment by the rebels on his remarks.

The rebel forces, who oppose the pro-European leadership in the capital Kiev, earlier confirmed fighting was under way.

They said five of their number had been killed.

Mariupol is Ukraine's largest Azov Sea port, and is important for steel exports.