Six servicemen have been killed and 12 wounded in eastern Ukraine despite a ceasefire deal, as both sides in the conflict accused the other of increasing attacks and building up weapons on the frontline.

The mutual recriminations have put further strain on a two-month-old truce between Ukraine's military and pro-Russian rebels, though it remains technically in force despite almost daily reports of casualties.

Ukrainian military spokesman Andriy Lysenko said: "The situation in the conflict zone remains unstable. Armed attacks by the enemy are taking place in almost all parts of the frontline."

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The death toll is the highest daily total since the February ceasefire was signed.

Mr Lysenko also reported civilian casualties, including two teenagers who were wounded in shelling in the Horlivka area, north of Donetsk.

Russia and Ukraine agreed in Berlin late on Monday to call for the pullback of smaller-calibre weapons from the front lines.

The most active exchange of fire was around villages near the rebel-held city of Donetsk.

The Donetsk regional administration, which remains in government hands, said it had tracked 30-40 vehicles carrying "Russian military equipment" heading in the direction of Popasna, a town on the frontline west of rebel-held Luhansk.

Meanwhile, the leader of the self-proclaimed Donetsk People's Republic (DNR), Alexander Zakharchenko, said Ukraine had brought heavy weapons close to the city.

Foreign ministers from Germany, France, Russia and Ukraine condemned the escalation of violence in a late-night meeting on the implementation of the ceasefire on Monday.

German foreign minister Frank-Walter Steinmeier, who hosted the meeting, insisted afterwards that the parties had no alternative but to abide by agreements forged in the Belarusian capital Minsk in February and September.

"Everyone knows that we have a long path ahead of us," he said. "But we're going to do everything we can to continue this process."

He said the new deal calls for the withdrawal of mortars and heavy weapons below 100mm (3.94in) calibre, as well as all types of tanks.

Mr Steinmeier said the four diplomats also agreed on the need to establish four working groups to address security issues, and the process for holding a local election in rebel-occupied areas.

The groups will also attempt to restart the exchange of prisoners of war, and to improve the dire economic situation in eastern Ukraine.

Russian foreign minister Sergey Lavrov said the parties were unable to agree on Ukraine's demands for the deployment of peacekeepers.

He also criticised laws passed recently by the Ukrainian parliament, which he said were eroding the Minsk deal.

"We underlined the need to fulfil the Minsk agreements in their entirety, not just in the military segment, but also in political, economic and humanitarian spheres," Mr Lavrov said.

More than 6,000 people have been killed since fighting started last April when pro-Russian rebels opposed to a new pro-Western government declared independence from Kiev.