ARTEMIVSK, Ukraine — Ukrainian forces fought their way out of the embattled town of Debaltseve in the early hours of Wednesday, choosing a risky overnight breakout rather than surrender as they abandoned the town to Russian-backed militants.

President Petro O. Poroshenko said in a televised statement that he had ordered the retreat from Debaltseve, a strategic transportation hub where intense fighting raged in recent days despite a cease-fire agreement signed last week in Minsk, Belarus.

Mr. Poroshenko sought to cast the retreat in a positive light, but the loss of the town was clearly a devastating setback for the army at the hands of the separatists. Still, by avoiding capture, the soldiers who made it out also avoided handing the rebels a powerful bargaining chip.

Separatist leaders have insisted that the cease-fire agreement did not apply to Debaltseve, but no exceptions were mentioned when the deal was announced in Minsk.

Mr. Poroshenko’s decision, and his earlier refusal to hand over the town during the cease-fire talks, cost the Ukrainian army an unknown number of casualties. As the scale of the nighttime fighting around the town comes into focus, those decisions could prove contentious in Ukraine.

In a post on Twitter and in the televised statement from an airfield in Kiev, the Ukrainian capital, before leaving to visit the front line, Mr. Poroshenko called the withdrawal “planned and organized” and said Ukrainian troops had accomplished their mission.

As many as 8,000 Ukrainian soldiers were said to be in Debaltseve before the withdrawal. It was unclear on Wednesday how many survived and avoided capture. Mr. Poroshenko said 80 percent of the army’s units had left.

By midday on Wednesday, limping and exhausted soldiers were showing up on the Ukrainian side of the front lines in the conflict, describing a harrowing ordeal that began with a surprise 1 a.m. order to retreat.

“Many trucks left, and only a few arrived,” said one soldier, who offered only his rank (sergeant) and his given name (Volodomyr) as he knelt on the sidewalk smoking. “A third of us made it, at most,” the soldier said.

Others said that a majority, at least, of the soldiers who set off from the town in a column of about 100 trucks had managed to escape the encirclement, many of them straggling out on foot after their vehicles were blown up.

The order to retreat was kept secret until the last minute, and soldiers were told to prepare in 10 minutes and pile into the beds of troop transport trucks, according to Albert Sardaryen, a 22-year-old medic who made the journey.

The trucks lined up on the edge of town, Mr. Sardaryen said, while tanks and tracked vehicles formed lines on either side of the truck convoy to try to shield the soldiers. The column drove through farm fields rather than use a main road that had been mined, and the trucks kept their headlights off to make them harder to spot.

The column came under attack almost immediately, he said, and trucks started breaking down and colliding in the dark. By dawn, the column was strung out on the plain and taking fire from all sides.

“They were shooting with tanks, rocket propelled grenades and sniper rifles,” and firing at the disintegrating column with rockets, he said. Dead and wounded soldiers were left on the snowy fields because there were too many of them to carry once the trucks were hit.

“We stabilized them, applied tourniquets, gave them pain killers and tried to put them in a place with better cover,” Mr. Sardaryen said of the wounded. Later, a Ukrainian unit from outside the encirclement drove in to try to retrieve the wounded, he said.

Mr. Sardaryen said he ran on foot for the final four miles or so. Many of the soldiers who made it out also did so on foot, though some trucks made it all the way through, he said.

Oleksandr I. Bogunov, an army private, said the order came to carry only what would be useful for the fight on the way out, and leave all other ammunition and weaponry behind.

Mr. Poroshenko’s order came after the separatists boasted of controlling the town on Tuesday, and after President Vladimir V. Putin of Russia suggested at a news conference in Hungary that Ukraine should accept its defeat at Debaltseve by the separatist forces, whom he described as “underdogs.” Russia is widely believed to be actively supporting the separatists.

“Life is life; it just goes on,” Mr. Putin said. “No need to dwell on it.”

Andriy Lysenko, a spokesman for the Ukrainian national security and defense council, confirmed the retreat from Debaltseve on at a briefing Wednesday afternoon in Kiev, the capital. He said the pullout was nearly completed.

“Today the armed forces of Ukraine are conducting the organized, planned retreat of units of forces of the antiterrorist operation from the city of Debaltseve,” Mr. Lysenko said. “At the moment, almost 80 percent of the Ukrainian units have retreated from this sector and this operation is to be completed soon.”

Though the cease-fire agreement, which was negotiated by Mr. Putin, Mr. Poroshenko, Chancellor Angela Merkel of Germany and the French president, François Hollande, was reached on Thursday, it did not formally take effect until Sunday. The leaders provided no real explanation of the decision to delay its implementation for about 60 hours.

Mr. Poroshenko has said that he was willing to accept an immediate halt in the fighting, and that the delay was at Russia’s insistence. That seemed to be a reflection of the advantageous position of separatist fighters on the ground in the battle for Debaltseve.

In any event, he delay provided a window for fierce and bloody combat, and when the cease-fire did take effect, it produced only a brief lull in the fighting.

Mr. Poroshenko spoke by telephone on Wednesday with Ms. Merkel and Vice President Joseph R. Biden Jr. about the continued fighting and the setback the Ukrainian forces had suffered.

Ukraine had asked its Western partners to apply diplomatic pressure to encourage the pro-Russian separatists to observe the cease-fire in Debaltseve, and to allow access for monitors from the Organization for Security and Cooperation in Europe. Talks on the issue among the organization, Russia and Ukraine ended without results around 9 p.m. on Tuesday, and the order for the retreat came four hours later.

Mr. Poroshenko urged the United Nations Security Council to prevent further breaches by Russia and the separatists. In a statement, the Ukrainian presidential administration said that Mr. Poroshenko and Ms. Merkel condemned the cease-fire violations in Debaltseve.

“It is a cynical attack on the Minsk agreements,” Mr. Poroshenko said in the statement. “Today, the world must stop the aggressor.”