SVITLODARSK, Ukraine (AP) — Ukrainian forces and Russia-backed separatists rocketed each other with heavy artillery fire Saturday — shelling that extended far beyond any front lines — as the hours ticked down to a cease-fire that was supposed to start at midnight.

Ukrainian President Petro Poroshenko declared he would consider imposing martial law if the fighting does not abate in the conflict that has killed more than 5,300 people since April.

The fighting centered around Debaltseve, a key government-held railway hub between the rebels' two main cities of Donetsk and Luhansk.

Shells rained down Saturday afternoon on the government-held town of Artemivsk, 40 kilometers (25 miles) north of Debaltseve, striking a school, which rapidly burned to the ground.

Associated Press reporters also saw an artillery barrage near the town of Svitlodarsk, 20 kilometers (12 miles) north of Debaltseve, as well as considerable movement of Ukrainian forces' armored vehicles and rocket launchers along the road.

An undated satellite image released Saturday by the Ukrainian government showed a 5-kilometer (3-mile) long cloud of black smoke hovering above Svitlodarsk, evidence of what it said was the scale of rebel shelling.

Russia has repeatedly denied repeated Western claims that it has sent troops and equipment to the rebels. But on Saturday, the U.S. ambassador to Ukraine, Geoffrey Pyatt, posted on Twitter what he said were satellite photos showing Russian artillery systems near the town of Lomuvatka, 20 kilometers (12 miles) northeast of Debaltseve.

The images could not immediately be verified.

Ahead of the cease-fire deadline, statements by separatist officials cast doubt on whether fighting would end at the appointed hour.

Alexander Zakharchenko, the rebel leader in the city of Donetsk, was quoted by the RIA Novosti news agency as saying his fighters would not allow Ukrainian forces to escape Debaltseve. Separatists have said the Ukrainian troops there would be offered only the opportunity to surrender.

Speaking in Kiev, Poroshenko accused separatists of mounting attacks of "threefold brutality" against government positions in Debaltseve and expressed concern that clashes could persist.

"I have warned before that if there is no peace, we may have to take the decision to impose martial law," he said. "In this event, martial law will be imposed not only in the Donetsk and Luhansk (regions), but across the whole country."

Under a peace deal reached Thursday, the warring sides are to cease firing at midnight (2200 GMT). But since a previous cease-fire in September failed to take hold and fighting escalated sharply in January, expectations for the new agreement are low.

The lack of a clear agreement over a line of division between the opposing forces, around Debaltseve in particular, seemed most likely to spark fresh tensions.

At midnight, each side is to pull heavy weaponry back from the front line, creating a zone roughly 30-85 miles wide, depending on the caliber of the weapons. The weapons withdrawals are to begin Monday and be completed in two weeks. No provisions are included for the withdrawal of troops.

Ukraine says Debaltseve should remain in government control under the terms of the September peace deal. A copy of that agreement leaked to Ukrainian media shows the town lying on the government side of the line of division agreed to by both the rebels and Ukrainian officials.

But even as Thursday's peace deal was announced, Russian President Vladimir Putin and Poroshenko appeared to disagree over the town's future. Putin said the rebels consider the Ukrainian forces there surrounded and expect them to surrender.

The peace plan also requires the Ukrainian government to resume paying pensions and state benefits to citizens on rebel-held territory. Ukraine's financial blockade against the rebels has led to a catastrophic collapse in living standards in eastern Ukraine, has deprived the poorest of any immediate means of support.

Officials in the key government-held port city of Mariupol, meanwhile, reported an array of artillery attacks hitting areas near the city Saturday morning.

Mariupol is on the Azov Sea and concerns are strong that the Russian-backed separatists aim to seize it to create a land corridor that runs between mainland Russia and the Crimean Peninsula, which Russia annexed 11 months ago.

The fighting started in April after armed separatists took control of towns and buildings in the Donetsk and Luhansk regions, fearing the new government in Kiev would suppress the heavily ethnic Russian population in eastern Ukraine.

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Leonard reported from Kiev, Ukraine. Jim Heintz in Moscow also contributed to this report.