Surging fighting around hotspots in separatist-held east Ukraine put fresh pressure on a shaky ceasefire under review at talks on Monday between the French, German, Russian and Ukrainian foreign ministers.

The four meet in Berlin from 1745 GMT to assess progress in the Minsk truce deal agreed in February between the Ukraine government and pro-Russian rebels to end a conflict that has killed more than 6,000 people over the past year.

At stake in the longterm for Moscow is the easing of punishing Western sanctions introduced as separatist insurgents allegedly operating with Russian military assistance seized control of a swathe of territory in the east.

"The ball is in Moscow's court," German Foreign Minister Frank-Walter Steinmeier said in an interview in Monday's Die Welt newspaper. "The sanctions are conditional on implementing the agreements."

"The situation remains fragile," he added, "but it is worth continuing, which is why we need to see Kiev and Moscow ready to work."
NATO however believes Russia has supplied more troops and weapons to the rebels, an unnamed alliance official was quoted as saying at the weekend by German paper,  Frankfurter Allgemeine Zeitung.

After weeks in which the ceasefire agreed in the Belarussian capital appeared to be largely holding despite isolated skirmishes, clashes seemed to be mounting in flashpoint areas.

OSCE monitors reported "renewed intensive fighting" Sunday around the rebel bastion city of Donetsk, with more than 1,000 explosions in under six hours, mainly artillery and mortar shell strikes.

In accordance with the Minsk deal the two sides claim to have withdrawn heavy arms from the frontline but the OSCE said "weapons with a calibre larger than 100mm were used by both sides during the fighting".

Around Shyrokyne, near the strategic port of Mariupol, the largest remaining city in the conflict zone still in government hands, the OSCE said dozens of mortar rounds were traded Sunday and heavy artillery fire from rebels "shook buildings across Mariupol".

- Call for peacekeepers -
The Organisation for Security and Cooperation in Europe, which has some 400 civilian monitors on the ground, complains that both sides are intimidating or restricting their movements in east Ukraine.

The OSCE parliamentary assembly's Finnish president Ilkka Kanerva said last week he was "convinced of the need to seriously consider an international peacekeeping mission".

Ukraine, which has repeatedly called for an international peacekeeping force, is expected to bring it up at the Berlin talks despite Moscow's resistance.

"We will do everything we can to convince our Western partners and the Russians of the need for a peacekeeping force," said Boris Lozhkin, chief of staff to Ukraine's President Petro Poroshenko.
"Contrary to the OSCE, peacekeepers are armed and can respond in case of attack," he added, saying a force of "thousands" of troops would be needed.

Ukraine's Foreign Minister Pavlo Klimkin also wants to see the release of hundreds of prisoners Kiev says are held by the rebels, one of the key points agreed in the Minsk deal.

Steinmeier said Monday's talks must move on to look at political problems between the two sides.

"We expect both Moscow and Kiev to seize the central issue of the implementation of the next phase of Minsk," Steinmeier told Die Welt.

The next phase foresees "the preparation of local elections in the areas occupied by the separatists, but also humanitarian access and reconstruction in eastern Ukraine," Steinmeier said.

The results of the talks in Berlin will be discussed at a meeting of G7 foreign ministers in Germany on Tuesday and Wednesday.