The leaders of France, Germany, Russia and Ukraine were locked in peace talks late into the night in Belarus, even as pro-Russian separatists launched some of the Ukraine conflict's worst fighting.
The Ukrainian army said 19 of its soldiers were killed at a single location near the railway hub of Debaltseve, some of the worst losses it has reported in nine months of war.
Rebels 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".
The summit is being held in Minsk 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.
Mr Putin arrived about an hour after the other three, who were due to meet briefly before he would join them.
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.
Earlier today, Moscow said there had been "noticeable progress" in talks ahead of the summit, but it still opposes Ukraine regaining control of its border - a key point in negotiations.
"Experts are working, there is noticeable progress," Russian Foreign Minister Sergei Lavrov told reporters ahead of the talks aimed at finding a way out of ten months of fighting between pro-Russian separatists and Ukraine's government.
But Mr Lavrov stressed that the issue of establishing border control should not be a pre-condition to any peace agreement.
"While there is fighting, while many other issues remain unsolved, this will be unrealistic, in my opinion," Mr Lavrov told reporters after talks with visiting Greek Foreign Minister Nikos Kotzias.
EU foreign affairs head Federica Mogherini said the summit in Minsk would be a turning point in the conflict.
"It is something (that) is going to be a turning point for good or for bad... if tonight in Minsk we manage to reach an agreement," Ms Mogherini said.
In the rebel stronghold of Donetsk, city officials said that eight people had been killed, including five in a mortar strike on the city during this morning's rush hour.
Rebels, who rarely give a military toll, said yesterday they had lost seven fighters.
Mr Hollande has pledged to "try everything right to the end" in order to resolve the conflict.
European officials say it is difficult to imagine the rebels agreeing to halt and go back to earlier positions after weeks during which they have been advancing relentlessly.
A Russian source quoted by the state RIA news agency said there were no plans to sign a document to resolve the conflict at the peace talks, and the main subject would be creation of a demilitarised zone.
Ukraine - Timeline of a Conflict
The war and years of endemic corruption have nearly bankrupted Ukraine, where the currency collapsed last week.
Ukraine is negotiating a rescue package with the International Monetary Fund and sources said that could be expanded dramatically to provide as much as $40 billion in aid.
The White House said US President Barack Obama urged Mr Putin in a phone call yesterday to agree to a peace deal.
The Kremlin, in its statement about the call, said Mr Putin and Mr Obama highlighted the need for the need for a political solution to the "internal" conflict in Ukraine.
The Kremlin also said the two leaders also noted the necessity to safeguard the rights of inhabitants of all Ukrainian regions, including Russian-speakers in the east.
