By Alexander Kolyandr in Moscow and Nick Shchetko in Minsk, Belarus 

Ukrainian forces and Russia-backed separatists clashed Saturday in eastern Ukraine, less than half a day before a cease-fire brokered by European leaders was due to come into effect.

The truce, seen as a possible last chance to put an end to a surge in the deadly fighting, is set to begin at midnight local time. But a previous deal in September collapsed, and doubts about the implementation of the new agreement grew as both sides squabbled over the specifics and violence continued.

The transport hub of Debaltseve remained at the center of severe clashes Saturday as separatists sought to encircle and seize the Ukrainian-held town. The cease-fire agreement technically puts the city under Ukrainian control.

"The militants are destroying the town of Debaltseve. Artillery strikes non-stop at civilian houses and buildings. The town is burning," said a post Saturday on the Facebook account of Vyacheslav Abroskin, the Ukrainian regional police chief for the embattled Donetsk region.

Ukraine army spokesman Col. Andriy Lysenko said the rebels were amassing heavy arms, and with the help of the Russian forces were trying to enlarge their territory ahead of the cease-fire.

U.S. State Department spokeswoman Jen Psaki said Friday the Russian military had deployed artillery around Debaltseve and was shelling Ukrainian positions. "We are confident these are Russian military, not separatist systems," she said in a briefing.

"This is clearly not in the spirit of this week's agreement," Ms. Psaki told reporters.

The U.S. ambassador to Ukraine, Geoffrey Pyatt, posted three satellite images on Twitter showing what he said were Russian artillery systems northeast of Debaltseve.

The Russian defense ministry denied the allegations, telling local news agencies that the "dark spots" on the pictures weren't proof of anything.

Ukrainian forces held their ground in Debaltseve despite the "insane pressure, shellings and several powerful attacks," Col. Lysenko said.

Heavy fighting was also reported near the Azov sea port of Mariupol. The city council said there were eight occasions of heavy shelling in the vicinity Saturday.

In Shyrokine, a town close to Mariupol, there were heavy battles since early morning on Saturday, Azov Battalion reported on its Facebook page. The Azov Battalion, a voluntary militia group fighting along with the Ukrainian army, said Shyrokine was nearly leveled by artillery fire. The group reported a large number of wounded and called for reinforcements.

In the past 24 hours, government forces had seven dead and 23 wounded, according to Col. Lysenko.

The rebels didn't provide a number of casualties. The head of the self-proclaimed Donetsk People's Republic, Alexander Zakharchenko, said Saturday the rebels under his command had stopped fire on all directions apart from Debaltseve, as "there was no mentioning of the town in the Minsk agreements."

Speaking in Donetsk, he added the whole region should be under the rebels' control in the future. The rebels control about a half of the territory of Donetsk region.

Ukraine President Petro Poroshenko put all the responsibility for any possible breach of the cease-fire on the separatists. "We are at the cross-roads now: Either the enemy stop fire, and the de-escalation and the political process start, or the enemy impose the conflict escalation on us and the whole world," he said Saturday. He reiterated that if the cease-fire doesn't hold, martial law will be imposed nationwide in Ukraine.

Disagreements also flared Friday over the deal, a further sign of the mistrust that could derail it.

Ukrainian Foreign Minister Pavlo Klimkin told parliament an amnesty outlined in the cease-fire wouldn't apply to the rebel leaders because a law passed in September excluded rebels responsible for "grave crimes."

Vladislav Deynego, a rebel leader in the Luhansk region, said that announcement showed Kiev was already backtracking. "Kiev is simply trying to dodge implementation of what we agreed to," Mr. Deynego said, according to Interfax.

The Ukrainian foreign minister also said he had begun work to involve the Organization for Security and Cooperation in Europe in plans for eventual local elections on separatist-held territory under Ukrainian law.

Donetsk rebel official Denis Pushilin told a separatist news agency that such consultations shouldn't be decided unilaterally. He noted that the pact calls for a dialogue on the local elections to start only after the removal of heavy artillery from the conflict zone, and after discussions with the separatist republics.

Oversight of the revived cease-fire falls to the OSCE, but questions have arisen as to whether it has enough manpower and expertise for the task.

A spokesman for the OSCE mission in Ukraine said Friday the organization was already enhancing its monitoring capabilities. In addition to increasing the size of its staff, he said the organization was exploring a variety of technologies, including satellite imaging.

Write to Alexander Kolyandr at Alexander.Kolyandr@wsj.com

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