By Aleksandar Vasovic and Andrei Makhovsky

DONETSK, Ukraine/MINSK (Reuters) - Civilians were killed on both sides in heavy fighting in eastern Ukraine on Friday, while an attempt to reopen peace talks in neighboring Belarus was aborted before it began.

Rebel delegates flew to the Belarus capital Minsk, only to announce that no talks would take place on Friday and they were flying straight back to Moscow. Any talks would be the first since a five-month-old ceasefire collapsed with a new rebel advance last week.

The main rebel stronghold Donetsk echoed to the sound of heavy artillery fire, including salvoes from multiple rocket launchers and heavier thuds from artillery coming from the direction of the airport, a constant battlefield.

A Reuters cameraman in Donetsk saw four covered bodies near a cultural center that had been hit by artillery while residents were queuing outside for humanitarian aid. A fifth body lay in a badly-damaged car nearby. A woman was weeping by one of the bodies. A kilometer (half mile) away, a sixth dead person lay where a trolleybus had been hit.

The separatists said the total death toll in those two strikes was seven, blaming government forces. Kiev said the shelling was carried out by the rebels to ruin the chance of peace talks. Both sides have made similar allegations throughout the conflict, which are impossible to verify.

"We are already used to this artillery and there's nothing we can do about it. Our boys are defending us," said Alla, a shopkeeper in downtown Donetsk.

In Debaltseve, east of Donetsk, seven civilians were killed on Friday by separatist shelling of their homes, regional police chief Vyacheslav Abroskin said in a Facebook post. Earlier he reported another seven civilians killed in and around the town in the previous 24 hours. The government-held town is a key rail and road junction in the east. It and nearby Vuhlehirsk have come under fierce attack from rebels encircling government garrisons there, with water and electricity supplies cut off.

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In the nearby rebel-held frontline town of Horlivka, eight civilians were killed in rocket attacks the previous day, the mayor's office said. Ukraine authorities did not comment. Kiev's military said five of its servicemen had also been killed and 23 wounded in fighting in the past 24 hours, describing the situation in the conflict zone as "hard".

"They are repeatedly using Grad (missiles), artillery, mortars, tanks and rocket launchers," spokesman Andriy Lysenko said in a televised briefing.

The past week has seen by far the worst fighting since the ceasefire was signed five months ago, with rebels announcing an offensive that Kiev says amounts to a repudiation of the truce.

NATO and Kiev accuse Russia of sending thousands of troops to support the rebel advance with heavy weapons and tanks. Moscow denies it is directly involved in fighting over territory that the Kremlin refers to as "New Russia".

European Union foreign ministers agreed at an emergency meeting on Thursday to extend for another six months economic sanctions against Russia that had been due to expire soon. Washington has promised to tighten its own sanctions, which have helped feed an economic crisis in Russia.

The arrival of two rebel negotiators in Minsk was the first sign of a reopening of negotiations since the rebels launched their latest advance.

But neither Kiev nor Moscow confirmed that they were ready for talks, and one of the rebel delegates, Denis Pushilin, swiftly announced they were heading back to Moscow. He said the rebels were prepared to press on with their offensive and seize more territory if artillery continues to fall on their cities.

"If shelling resumes, then we reserve for ourselves the right to continue the offensive and go to the very borders of Donetsk and Luhansk regions," he said, referring to the two provinces where separatists have declared "people's republics".

The Ukrainian foreign ministry said it was ready to participate in talks either Friday or Saturday but was waiting for an agreement on draft documents.

FEAR OF ASSAULT

The immediate fear of Kiev and its NATO allies is of a rebel offensive on Mariupol, with 500,000 people by far the biggest government-held city in the two restive provinces. It was hit by shelling on Saturday which Kiev said killed 30 civilians, although the rebels have since denied it is a target for now. They halted at its gates during their last big advance five months ago.

The rebels have said their principal aims for now are to push government guns out of range of their cities and make their positions more secure by "straightening out the front" - choking off a government-held pocket around Debaltseve.

Both are moves that would make existing rebel areas more defensible if, as many Western countries suspect, Moscow's aim is to pursue a stable "frozen conflict" in eastern Ukraine.

A rebel assault on Mariupol, with the potential to unleash unprecedented urban warfare, is a far more dangerous prospect. While the rebels say they are not trying to capture it yet, they have repeatedly said they reserve the right to do so.

The leaders of France and Poland, two of Europe's strongest advocates for tighter sanctions against Russia, met in Paris and called for EU relations with Moscow to be "rethought".

Donetsk, a city of a million people before the war, has become a desolate frontline town.

"I have nothing to sell to leave. I am a pensioner and if I have to die here, so be it," said Leonid, in his 70s, dressed in a shabby winter coat and leaning on a cane. He and his wife moved in with relatives after their apartment was destroyed a month ago. He has received no pension since Ukraine cut off funding three months ago.

Dozens of cars and trucks queued at a Ukrainian checkpoint outside Donetsk, where a crowd of people were filling in applications to enter government-held territory. "All civilians should be evacuated, but neither (the rebels) nor the Ukrainians could care less, and we are dying every day," said schoolteacher Martina Alexandrovna, 46.

(Additional reporting by Natalia Zinets in Kiev; Writing by Alessandra Prentice, Richard Balmforth and Peter Graff)