Russian-backed separatists in eastern Ukraine accused government forces on Thursday of opening fire on their territory four times in the past 24 hours and said they were trying to establish if anyone had been hurt or killed.
It was not immediately clear how serious the incidents were and there was no immediate reaction from the Organization for Security and Cooperation in Europe (OSCE), which has been monitoring the situation in eastern Ukraine but has pulled out some of its monitors in recent days.
Ukraine's military denied the separatists' accusations that government troops had attacked, saying it was the rebels who shelled the Ukrainian military.
Such incidents have occurred many times over the past eight years but this clash comes after Russia has massed more than 100,000 troops close to Ukraine's borders while demanding that NATO pledge not to accept Ukraine as a member.
The West has threatened Russia with new sanctions if it attacks Ukraine; Russia denies planning any attacks.
Any escalation in the years-long conflict between the Russian-backed separatists, who seized a swath of territory in eastern Ukraine in 2014, could fuel tension between Russia and the West.