At least 11 pro-government fighters were killed Saturday in an ambush by pro-Russian separatists, only a few hours after a ceasefire agreement between both sides came into effect, reported the Russian Interfax news agency.
Citing separatist sources, Interfax also reported that several rockets were landed in suburbs of separatists' bastion of Donetsk.
Ukraine and pro-Russian separatists reached a ceasefire agreement on Friday in the Belarussian capital of Minsk, the first step toward ending fighting in eastern Ukraine that has caused the worst standoff between Moscow and the West since the Cold War ended.
Fierce clashes erupted in Eastern Ukraine after the announcement of Crimea's independence and annexation to Russia last February.