When the pact was signed with a two-day window before the truce, some last-minute jockeying for position was expected.
But the intensity and scope of the violence raised concerns that the agreement signed this week, rife with ill-defined and ambiguous provisions, might prove as ineffective as the first cease-fire pact, signed in September.
Artillery shelling and gunfire reverberated in the area around Debaltseve, a strategic rail hub where rebels were said to have severed the last land route into town, leaving government forces surrounded.
At least 18 people were reported to have died there.
Fighting flared in many towns.
Two people were killed and six wounded in an artillery strike in the Ukrainian-controlled town of Shchastya, near rebel-held Luhansk.
A Ukrainian military spokesman, Colonel Andriy Lysenko, said 11 Ukrainian soldiers had been killed and 40 wounded in fighting after the cease-fire agreement.
Mr. Lysenko said separatists had fired artillery rounds at over 30 towns and villages in the east.
In a sign that the Ukrainian Army, too, was trying to land blows before midnight on Saturday, three children were reported killed by artillery fire in the rebel-held town of Horlivka.
Shells struck a hospital in Donetsk, a rebel spokesman said.
Some of the heaviest fighting broke out along a poorly defended, 31-mile Ukrainian supply route into Debaltseve within hours of the signing of the peace agreement.
Soldiers and medical crews interviewed at a hospital in Artemivsk said the rebels now controlled the road, and as evidence pointed to the ambulances and resupply trucks blown up by mines that now pepper a stretch of the route.
The Ukrainian military was reeling from the assault.
Rumbling over the snowy steppe, a line of rebel tanks assaulted one Ukrainian stronghold on the road, according to soldiers at a hospital in Artemivsk.
Soldiers hunkering down in pillboxes at positions along the road were trying to hold out until Saturday, Sgt. Valeri Dedkivsky said, adding that even then he had little faith the cease-fire would take effect as scheduled.
He stood Friday morning at the last Ukrainian checkpoint and gazed over a now mined and inaccessible stretch of the supply road, watching smoke swirl up from three burning Ukrainian military trucks.
“Our brothers are dying right now,” he said.
“This delay was not for us.”
In Washington, the State Department spokeswoman, Jen Psaki, called the involvement of Russian troops in the attack on Debaltseve a violation of the spirit of the cease-fire agreement.
“The Russian military has deployed a large amount of artillery and multiple rocket launcher systems around Debaltseve, where it is shelling Ukrainian positions,” Ms. Psaki said, reading a prepared statement.
“We are confident that these are Russian military, not separatist systems.”
Ms. Psaki said that the Russian military had moved air defense units near the town, and that the United States had reliable reports that Russia was preparing a large shipment of supplies to pro-Russian forces.
The Russians have insisted they are not assisting the separatists, and there were no firsthand reports of Russian troops engaged in battle.
But in a proxy war, NATO and Western analysts say, Russian troops have consistently operated in the background, supplying, training and guiding the rebels.
President Vladimir V. Putin of Russia, Chancellor Angela Merkel of Germany and President François Hollande of France joined President Petro O. Poroshenko of Ukraine in negotiating the truce agreement reached on Thursday in Minsk, Belarus — the second in the nearly year-old eastern Ukraine war.
At the time, American officials warned the Kremlin and the separatists not to try to seize new territory before the cease-fire takes effect.
“Any effort to grab more land between now and Saturday night will seriously undercut this agreement,” a senior Obama administration official told reporters.
Another senior Obama administration official, who spoke on the condition of anonymity, told reporters at the same Thursday briefing that the United States might increase sanctions or send defensive arms to the Ukrainians if the new Minsk agreement was violated by the Russian side.
While the new cease-fire agreement is considered fragile, it was welcomed Thursday as the best hope of resolving the conflict, which has left more than 5,350 people dead.
But rather than the hoped-for calm, the cease-fire appeared instead to cause a sharp escalation.
Mr. Poroshenko said the two-day delay in ending hostilities was a concession to the Russian-backed militants and Putin.
After the overnight talks, Putin said Mr. Poroshenko refused to acknowledge that the separatist forces had surrounded up to 8,000 Ukrainian soldiers in Debaltseve, but the Russian leader said he hoped that consultations between military commanders would settle that matter.
Mr. Poroshenko, speaking after the signing, said Ukraine had tried but failed during the 16 hours of talks to hold out for an immediate cease-fire.
“Unfortunately, they demanded that we give them a minimum of 70 hours before the start of the cease-fire,” he said.
As the Russian military and separatist forces have pressed the attack, Western officials have become increasingly worried that Putin is seeking to seize Debaltseve before the cease-fire begins, expose the weaknesses of Ukraine’s forces and deal a political blow to Mr. Poroshenko.
On the last point, Putin seemed to be succeeding in some quarters.
Oleg Lyashko, the leader of the Radical Party, which is part of the majority coalition in Ukraine’s Parliament, said that Mr. Poroshenko had made overly steep concessions to Putin that he described as a ticking “time bomb” that would give Russia a premise for resuming hostilities in the east.
“Let them bite your finger, they will bite your whole hand, and then your head,” Mr. Lyashko told colleagues.
“This is a well-known folk wisdom.”
In Artemivsk, a woman and a 7-year-old child were killed Friday afternoon when cluster munitions hit a neighborhood, exploding in rapid cadence and leaving behind piles of shattered glass, sagging electrical wires and residents standing about in horrified awe.
In the hospital courtyard, ambulance crews hurriedly wheeled about bloodied, freshly wounded soldiers.
Medical helicopters buzzed in and out through the day.
At a school, teachers herded children indoors when the booms of artillery started rattling windows.
“I don’t know what happened,” Alla G. Neschadym, a nurse at the Artemivsk Central Regional Hospital, said in an interview about the battle that she said began late Thursday afternoon.
“But I saw the results. The wounded came in all night long.”
The casualties flowed in from positions along the road.
Soldiers were wounded by shrapnel from mortar rounds and rockets, from gunfire and in explosions when their vehicles hit mines.
In one explosion, one soldier suffered damage to both eyes and “will probably be blind,” Ms. Neschadym said.
From Jan. 6 until Feb. 11, she said, the hospital treated 1,004 wounded soldiers.
Overnight Thursday to Friday after the cease-fire signing, doctors treated 97 wounded soldiers in a frantic, bloody scene.
By Friday, most of the wounded had been evacuated to the west, but a stack of bloody stretchers remained in the hospital hallway.
In Moscow, the office of Putin endorsed the cease-fire, while trying to distance the Kremlin from enforcing it.
Dmitry S. Peskov, the spokesman for Putin, said that signing the cease-fire plan made Russia one of its guarantors, but repeated the standard Kremlin position that it could not affect developments on the ground.
“Russia is not a party which implements this set of measures,” Peskov was quoted as saying by RIA-Novosti, a state-run news agency.
“We simply cannot do this physically, because Russia is not a participant in this conflict.”
Source: The New York Times