There have been violent clashes in eastern Ukraine overnight – as seen in images posted on the internet.

The Kyiv authorities say pro-Russian activists attacked a base of the Ukrainian national guard at Mariupol. In the gun battle that ensued, three attackers were killed.

A local news website spoke on Thursday morning of four deaths.

Ukraine’s interior ministry said earlier that armed protesters demanded that soldiers switch sides and back their revolt. The troops refused.

The government said a major security operation was underway in the area. Mariupol’s city administration was seized by rebels at the weekend. It is not known if it is still under their control.

The Interior Minister Arsen Avakov claimed on Facebook that 13 of those who assaulted the base were wounded and more than 60 detained.

The Ukrainian TV channel Hromadske says its film shows armed activists challenging troops to give up weapons.

If the deaths are confirmed, the event is the bloodiest so far in the 10-day-old pro-Russian revolt in eastern Ukraine.