Ukraine: The leaders of France, Germany, Russia and Ukraine were due to attend a peace summit on Wednesday, with Ukraine’s pro-Moscow separatists tightening the pressure on Kiev by launching some of the war’s worst fighting in an assault on a government garrison.

The Kiev army said 19 of its soldiers were killed in a day of pro-Russian separatist assaults at a single location near the railway hub of Debaltseve, some of the worst losses it has reported in nine months of war.

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.

The summit is being held in neighbouring Belarus under a Franco-German proposal to try to halt the fighting. Chancellor Angela Merkel and President Francois Hollande will meet Ukraine’s Petro Poroshenko and Russia’s Vladimir Putin.

Hopes for a breakthrough appear slim and would depend on Ukraine making most of the concessions, with advancing rebels unlikely to agree to halt and go back to previous positions.

A surge in fighting in the 24 hours before the leaders were to gather, including a rocket attack that killed 11 people deep in government-held territory on Tuesday, could be intended to force Poroshenko to accept a deal recognising the rebel advance.

That all four leaders would attend was only firmly confirmed on the final day.

“There are a number of problems which remain to be resolved ... but it is very likely to go ahead,” French Foreign Minister Laurent Fabius told France Inter radio. “It is really a last-chance negotiation.”

Still, Moscow expressed optimism. A Russian diplomatic source said it was 70 percent likely that an agreement would be reached.

“The presidents aren’t travelling (to Minsk) for no reason,” the source said.

Foreign Minister Sergei Lavrov said there had been progress in the run-up to the summit but Kiev could be holding back a deal by insisting on control of the Russian-Ukrainian border, part of which is held by the separatists.

The talks are taking place while an International Monetary Fund mission is trying to negotiate a bailout to save Ukraine from bankruptcy brought on by corruption and mismanagement as well as war. Prime Minister Arseny Yatseniuk said the talks were tough but he hoped for a loan deal within days.