"The situation was tense in the area of Mariupol. The militants fired 152mm howitzers and 120mm mortars on the villages of Shyrokyne and Berdianske," the headquarters said.

In the evening, the militants also fired 120mm mortars on the village of Pisky and the mine of Butivka, as well as 122mm mortars near the village of Hranitne, and 120mm and 122mm on the village of Novotoshkivka in Luhansk region.

Weapons of caliber above 100mm were to be withdrawn out of range of the front line weeks ago according to the Minsk peace agreements. However, there have been reports of both sides using such weapons in recent clashes. Ukraine informed the OSCE last week that it was moving heavy weapons forward to fight off a major attack by Russian-backed militants on the town of Maryinka last week.

The OSCE also noted the movement of heavy weapons in militant-held parts of Ukraine ahead of that attack, and due to militant restrictions on its movements is unable to monitor large parts of militant-held areas, including a swathe of land around a major railway line leading directly up to one of the most active parts of the front line.

"A military clash between Ukrainian troops and a sabotage and reconnaissance group of the enemy took place near the village of Syze in the evening. As a result of battle the enemy had to retreat. There were no casualties among the Ukrainian army," the report reads.

In addition, the militants fired small arms and rocket-propelled grenades in the area of Artemivsk, near the towns of Maiorsk and Zolote, and the villages of Leninske and Troitske. They also fired tanks and howitzers on the villages of Vodiane and Trudivske at night.

"By morning, the militants fired 122mm artillery systems on the villages of Starohnativka, Novohryhorivka and Hranitne in Donetsk region," the report says.

"During this period, the Ukrainian army recorded ten reconnaissance overflights by enemy drones, mainly in Luhansk region and in the area of Artemivsk," the ATO headquarters said.