Avdiivka (Ukraine) (AFP) - Ukrainian forces and Russian-backed rebels were on Tuesday locked in fighting for a third straight day at a flashpoint town that left thousands shivering without power and sparked renewed EU concern about security in its backyard.

The industrial hub of Avdiivka came under an unexpected assault Sunday from insurgents seeking to wrest back territory controlled by Kiev during the nearly three-year war.

The clashes have claimed the lives of at least 13 civilians and fighters on both sides since Sunday -- the worst outburst of violence since the two sides agreed a new truce on December 23.

Ukrainian President Petro Poroshenko underscored the urgency of the situation by cutting short a visit to Berlin on Monday and convened an emergency meeting of his National Security and Defence Council.

Poroshenko is worried that Donald Trump's rise to the US presidency and praise for Russia's Vladimir Putin may add fuel to a conflict that began shortly after Ukraine's 2014 ouster of its Moscow-backed leader and tilt toward the West.

Putin's spokesman Dmitry Peskov told reporters the Kremlin was "extremely worried" but had "reliable information" that renegade units of pro-Kiev fighters were in fact responsible for the initial attacks.

An AFP reporter saw the separatists shell the town of about 20,000 people with repeated rounds of Grad multiple rocket systems and artillery fire from the early morning.

"Right now, there is no power. We have not resolved problems with heating homes, and the gas pipe has been shattered," local Ukrainian army unit spokeswoman Olena Mokrynchuk told AFP.

The town's military administrator Freedon Vekua told AFP he was preparing for a possible evacuation of the town that sits just north of the rebels' de facto capital of Donetsk because of the power outage.

Ukraine is struggling through freezing conditions in which temperatures drop to -20 degrees Celsius (-4 degrees Fahrenheit) at night.

"The issue of an evacuation has not been decided fully. We see it as our very last resort because there is still a chance of restoring heating," said Vekua.

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The town's heating is provided by a coke plant that has been heavily damaged by the falling shells.

Plant director Musa Magomedov said it would be incredibly difficult to resume gas production were the factory's generators shut down.

That would leave Avdiivka without a source of local power and uncertainty about its future. The fighting has prevented repairs being carried out, Magomedov said.

- Global condemnation and concern -

The Organization for Security and Cooperation and Europe (OSCE) is responsible for monitoring ceasefire violations and organising peace talks between envoys from Russia and Ukraine.

It helped negotiate a February 2014 truce deal in Minsk that was co-sponsored by Germany and France and which EU leaders cling on to as the one remaining roadmap to peace.

"The intense fighting around Avdiivka in the last few days... is a blatant violation of the ceasefire, as stipulated by the Minsk agreements," the EU foreign affairs arm said in a statement.

The OSCE said the fighting was "of grave concern" while the US embassy tweeted that it was especially worried about "the 2,500 children who are without water, electricity and heat".

Charge d'affaires Kate Byrnes of the US mission to the OSCE's Permanent Council said that "Russia and the separatists initiated the violence in Avdiivka".

"We call on Russia to stop the violence, honour the ceasefire, withdraw heavy weapons, and end attempts to seize new territory beyond the line of contact," she said in an address to the OSCE Permanent Council in Vienna.

The conflict has killed nearly 10,000 people since 2014 -- more than half of them civilians -- and plunged Moscow's relations with the West to a post-Cold War low.

The Kremlin denies backing the insurgents and only admits that Russian "volunteers" and off-duty soldiers have entered the warzone of their own free will.

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