The Russian defence ministry said that its positions had come under attack in the Luhansk, Donetsk and Zaporizhzhia regions, but that its troops were observing the ceasefire.

"Four mortar shells were fired at Russian positions by the Armed Forces of Ukraine in the direction of Lyman," it said.

Reuters could not immediately establish if there had been any lull in fighting. Kyiv has said it has no intention of halting fighting for the proposed Russian ceasefire, which Ukraine and its Western allies called a ploy designed to give Moscow time to reinforce troops and equipment.

Putin ordered the 36-hour ceasefire in the 10-month-long war in a surprise move on Thursday, saying it would mark the Russian Orthodox Christian Christmas.

(Reporting by Reuters; Editing by Mark Heinrich and Peter Graff)