The Organization for Security and Co-operation in Europe Special Monitoring Mission to Ukraine (OSCE SMM) says heavy artillery, tanks, and 120mm mortars were used during intensive fighting near Donetsk on April 12, when monitors recorded over 1,100 explosions.

"In total, between 09:35 and 13:10 [on April 12], the SMM heard 1,166 explosions caused by different weapons from the south-west, west, west-north-west, north-west, north and north-north-east, in a distance of approximately 2 to 5km of the SMM's position," the SMM said on Tuesday in a report based on information received as of April 13.

"In the SMM's assessment, approximately 30 per cent of the total number of explosions was caused by heavy artillery, tank and 120mm mortar fire and the remaining by anti-aircraft machine gun, heavy-machine gun and small arms," it said.

"During the day, the SMM observed several ambulances, travelling with emergency lights and sirens on, moving from the airport to the city centre," it said.