European leaders met into the early hours of Thursday in a grinding, last-ditch effort to forge a cease-fire in Ukraine's increasingly violent year-old conflict, which has brought warfare not seen in years to Europe.

Amid some of the heaviest fighting to date in eastern Ukraine, the leaders of Ukraine, Russia, Germany and France were haggling over a deal to stop the fighting between pro-Russian rebels and pro-government forces. Top officials described the extraordinary summit as the final chance to avert an even harsher escalation.

"The entire world is waiting to see whether the situation moves toward de-escalation, weapons pullback, cease-fire, or spins out of control," Ukrainian President Petro Poroshenko said in the Belarusan capital, Minsk, ahead of the negotiations. He told his cabinet that he was poised to declare martial law if the talks failed.

Leaders have said they were working from the outlines of a September cease-fire deal that was never fully implemented and that evaporated completely in recent weeks. The death toll has more than doubled since then, to more than 5,400, according to U.N. estimates, and rebels have captured hundreds of square miles of additional territory.

In rough terms, any deal will require both sides to stop shooting and for heavy weaponry to be pulled back from the front lines within days to create a demilitarized zone. Rebel-held territories have demanded deep autonomy from the Ukrainian central government in Kiev. Points of contention included what form of autonomy and how to define the borders of rebel-held territory, as the battle lines have moved sharply since last summer.

Kiev has also demanded that Russia stop the cross-border flow of weapons and fighters into Ukraine.

The conflict has brought relations between Russia and the West to lows not seen since the Cold War. Within Ukraine, Poroshenko has been forced to give up dreams of leading a unified nation under the full control of Kiev. He has set up hard internal borders and cut off people, goods and money flowing between Kiev-held and rebel-held territories.

But European leaders have tempered expectations about the outcome.

"A ray of hope, but not more," said Steffen Seibert, a spokesman for German Chancellor Angela Merkel, in Berlin before the talks started.

Russian Foreign Minister Sergei Lavrov said late Wednesday that the talks were going well but that a deal may not be ready to be signed until Thursday morning.

Russia wants guarantees that Ukraine will not tilt westward and join alliances such as NATO. Russian President Vladimir Putin said fears of that possibility drove his annexation of Ukraine's Crimean Peninsula last March, a decision that set off the new conflict between Russia and the West.

Anger in the West toward Russia has grown since the fighting reignited last month, and the White House has been considering whether to send arms to the Ukrainian military. That would bolster Ukraine's chances against an opponent that the West says is supplied by Russia -- but it would also run the risk of triggering a fierce proxy war with the Kremlin on Ukrainian soil.

Merkel and French President Francois Hollande have said they are against arming Ukraine, and they flew last week on a surprise mission to Moscow in an attempt to jump-start the peace talks.

Earlier this week, President Barack Obama called Putin to warn that if he did not stop supporting the rebels and come to terms on a peace deal, "the costs for Russia will rise." Obama said separately that his decision about arming Ukraine would depend in part on the outcome of the talks.

Russia has hotly denied arming the rebels or ordering troops into Ukraine, saying that the Russian citizens who are fighting there are doing so as volunteers.

Fighting in recent days has come close to the intensity during the height of the conflict, in August, as both pro-Russian rebels and Kiev forces battle bitterly to expand their territory.

A top U.S. military official said Wednesday that Russian troops were currently fighting in Ukraine's east, and that Russia has massed thousands of additional troops on its border with Ukraine. That menacing signal has come at other tough points in the conflict.

"It's very obvious from the amount of ammunition, type of equipment, there's direct Russian military intervention" near Debaltseve, a key transport hub where some of the most intense fighting has taken place in recent days, the U.S. Army's Europe commander, Lt. Gen. Ben Hodges, said Wednesday. He said the United States plans to continue training Ukrainian troops in the western part of the country.

A Ukrainian military spokesman said Wednesday that 19 soldiers were killed and 78 wounded in the fighting near Debaltseve in the past day. The steep toll was comparable to the peak of fighting in August.

Separately, rebel officials said seven civilians were killed in their stronghold city of Donetsk when a minibus was shelled early in the morning.

Poroshenko rose to power on the back of a pro-European movement that wants to orient Ukraine firmly toward Europe and away from Russia. He faced harsh domestic criticism for agreeing to the September cease-fire deal because many in Kiev said he had traded away too much to the Kremlin. Any new bargain raises the prospect that he will come under even heavier political fire.

At the same time, Ukraine's economy is in tatters, and the nation is close to default. Its currency has shed two-thirds of its value in the last year. Its financing needs appear to outstrip what international lenders are willing to give it. And it has struggled to focus on economic reforms with the war raging in the east.