KIEV, April 17 -- Heavy casualties have been reported from a clash between armed pro-Russian activists and Ukrainian servicemen at a military base in the country's eastern Donetsk region early Thursday.

Ukraine Interior Minister Arsen Avakov said on his Facebook account that at least three rebels were killed and 13 others were injured when a group of 300 pro-Russian protesters attacked the military base in Mariupol city with Molotov cocktails and firearms.

Ukrainian servicemen had to respond, Avakov said, adding that the officers first fired warning shots against the attackers and then opened fire as the assault continued.

He said that there were no casualties among the servicemen and 63 attackers had been detained.

Meanwhile, the healthcare department at Donetsk regional administration said that two people were killed and 15 injured during the attack, without specifying the political affiliation of any of the victims.

The clash came just hours before top diplomats from Ukraine, Russia, the European Union and the United States prepared to meet in Geneva over the ongoing crisis in eastern Ukraine.

A new wave of unrest erupted in the country's east over the weekend, as pro-Moscow activists seized administrative buildings in several eastern cities, demanding a referendum on autonomy and closer ties with Russia.

Kiev has accused Russian intelligence forces of instigating the unrest, but Moscow strictly denied the accusations, saying it does not interfere in its neighbor's internal affairs.