AKIPRESS.COM - The leaders of France, Germany, Russia and Ukraine held peace talks in Belarus deep into the night, while in Ukraine pro-Moscow separatists tightened the pressure on Kiev by launching some of the war's worst fighting.

Ukraine's army said on Wednesday that 19 of its soldiers were killed in pro-Russian separatist assaults near the railway town of Debaltseve, some of the worst losses it has reported in nine months of war, reports Reuters.

Rebels who tore up a five-month-old truce in January are trying to encircle government forces in Debaltseve, a strategic location that would let them link up their main strongholds.

Fighting has already killed more than 5,000 people, and Washington is now openly talking of arming Ukraine to defend itself from "Russian aggression", raising the prospect of a proxy war in the heart of Europe between Cold War foes.

A surge in fighting in the 24 hours before the summit, including a rocket attack that killed 17 people in government-held territory on Tuesday, could be intended to force Ukraine to accept a deal recognising the rebel advance.

The summit was being held in neighbouring Belarus under a Franco-German proposal to try to halt the fighting. Chancellor Angela Merkel and President Francois Hollande held talks with Ukraine's Petro Poroshenko and Russia's Vladimir Putin that a Ukrainian presidential aide said could continue well into Thursday morning.

The leaders were planning to sign a joint declaration supporting Ukraine's territorial integrity and sovereignty, a Ukrainian delegation source said.

The source said a separate document would be prepared by a "contact group" of Russia, Ukraine and the Organization for Security and Cooperation in Europe affirming commitment to a ceasefire plan drawn up in Minsk last September and also signed by separatist leaders.

The comments were a partial read-out from the Ukrainian side and it was too early to say what compromise, if any, might be worked out between Ukraine and Russia.

The four leaders met alone at about 12.15 p.m. ET and then went into a full summit with their delegations.

Merkel, Poroshenko and Hollande were smiling when they entered the heavily decorated conference room, each of their places round the table marked by a small golden clock.

The talks could continue for at least another five to six hours, a Ukrainian presidential aide said early on Thursday, after the discussions had already been underway for seven hours.

"We've got another 5-6 hours of work. At least. But we should not leave here without an agreement on an unconditional ceasefire. There's a battle of nerves underway," aide Valeriy Chaly said in a Facebook post.