Three pro-Russian separatists were killed in South-Eastern Ukraine's Mariupol, Ukrainian Interior Minister Arsen Avakov said on Thursday.

Avakov added 13 activists were injured and 65 arrested in clashes with Ukrainian forces sent to conduct an "anti-terrorist" operation in response to recent riots.

The Minister made the announcement on his Facebook profile and was quoted by UNIAN news agency.

"Overnight, in response to the attack of military units of the National Guard in Mariupol, Interior Ministry forces carried out an operation to locate the attackers, which amounted to around 300 people," the message reads.

After Kiev's forces were pelted with landmines and Molotov cocktails, the military fired warning shots.

Donetsk region has been gripped by pro-Russian protests since last week, with activists seizing a number of governmental buildings in several cities and towns, demanding federalization.

Kiev announced an 'anti-terrorist operation' involving military units against the protesters.