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

Observers from the Organization for Security and Cooperation in Europe said Sunday that its mission observed an intense clash with the use of tanks and 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.

Rebel officials as well as Ukrainian and Russian colonels in charge of monitoring the cease-fire went early Monday afternoon to the northern outskirts of Donetsk, a scene of heavy fighting Sunday night.

The rebels told reporters they took captive a Ukrainian soldier and showed the body of another Ukrainian soldier. Intermittent shelling and exchanges of machine gun fire were heard from what appeared to be half a mile from the scene.

A rebel with the nom de guerre Monakh told The Associated Press that one rebel has been killed and five more injured in fighting in the north of Donetsk.

Oleksandr Motuzyanyk, military spokesman for the Ukrainian presidential administration, told reporters at noon on Monday that one Ukrainian soldier was killed and six wounded in the previous 24 hours.

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killed more than 6,000 but had largely subsided since the cease-fire was announced in February and some heavy weaponry withdrawn.

Both the OSCE and Ukrainian officials said they had witnessed shelling from heavy weaponry that was supposed to have been withdrawn from the front line.

Col. Andriy Lishchynskyi, a Ukrainian representative for monitoring the cease-fire in the east, blamed the clashes on "a highly emotional state and personal animosity" between the fighters on both sides, according to the Interfax news agency.

Rebels and government forces are still separated only by hundreds of meters at some sections of the front line.

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

Dmytro Kuleba, the Ukrainian foreign ministry's special envoy, in a Twitter post on Monday described the fighting in the area as "setting the stage for another act of a policy drama that will be playing in Berlin today."

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 over the weekend compared with previous weeks.