Fighting has picked up in eastern Ukraine, after more than a month of relative calm, as diplomats gathered in Berlin Monday to discuss the Ukraine crisis.

The military conflict between Russia-backed rebels and government forces has killed more than 6,000 but has largely subsided since the cease-fire was announced in February and at least some heavy weaponry withdrawn.

Observers from the Organization for Security and Cooperation in Europe said Sunday that its mission observed an intense clash with the use of tanks, heavy artillery as well as grenade launchers and mortars in the north of the rebel stronghold of Donetsk. On Sunday alone, the OSCE recorded at least 1,166 explosions, caused mainly by artillery and mortar shell strikes in northern Donetsk as well as on its outskirts including the airport, now obliterated by fighting

The OSCE also reported intense mortar fire outside the village of Shyrokyne by the Azov Sea but said its representatives were repeatedly barred from accessing the village on Sunday.

Mortar fire was also heard at night and in the morning on Monday in central Donetsk.

Foreign policy chiefs from Russia, Ukraine, France and Germany are meeting to discuss the crisis in Ukraine later on Monday.

In an indication that hostilities are picking up in the region, the rebel Donetsk News Agency reported on Monday that the number of injured fighters tripled on the weekend compared with previous weeks.