AVDIYIVKA, Ukraine/GENEVA (Reuters) - International security and aid organizations appealed to Ukraine and Russian-backed separatists on Friday to withdraw from front lines where fighting has dramatically escalated or risk a humanitarian disaster.

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Since Jan. 29, shelling on both sides of the front line near the government-held town of Avdiyivka has been heavier than at any time since last summer. That has refocused global attention on a simmering conflict that strained relations between Russia and the West to the worst degree since the Cold War era.

The February 2015 Minsk peace agreement only locked the two sides in a stalemate that has been broken periodically by sharp resurgences of fighting that Kiev and the Kremlin accuse each other of instigating.

“We have had many flare-ups before and yet somehow the sides have pulled back, reverting to an uneasy, often violent static confrontational stance,” said Alexander Hug, deputy head of the OSCE ceasefire monitoring mission in Ukraine.

“Now however the stakes are even higher, there is a potential humanitarian and ecological disaster about to unfold,” he said in a briefing via video link from eastern Ukraine. Monitors for the OSCE, Europe’s leading security and human rights watchdog, had recorded an “unprecedented” 11,000 explosions on Tuesday.

Civilians in both government- and separatist-held territory are especially at risk because damage to utilities infrastructure has left thousands with little or no access to electricity or water amidst subzero winter cold. Persistent fighting has hampered repair efforts.

A regional director for the International Committee of the Red Cross said the ICRC had recommended the creation of up to seven “safety zones” around key power and water infrastructure such as pumping stations close to the front line.

“What is proposed is that both sides should agree that those areas should not be subjected to military activity especially artillery shelling, which means there has to be an agreement on all sides and there has to be some form of withdrawal of military forces,” Patrick Vial told the briefing.

The U.N. human rights office warned of environmental danger posed by possible acid leaks from a phenol plant near the village of Novhorodske where the two sides’ combatants are only 400 meters apart.

Previous OSCE-backed efforts to have forces disengage from front lines have had mixed or no success.

Ukraine and Russia blame each other for the flare-up. Pro-Russian separatists said six civilians had been killed and 34 wounded on their territory since Sunday, while Ukraine has reported three civilians and 15 servicemen killed in government-held areas over the same period.

A Reuters witness in Avdiyivka said the intermittent boom of artillery fire could be heard in the distance throughout the day on Friday, although shelling was significantly less intense than during the previous night and earlier in the week.

In the separatist-held city of Donetsk, pensioner Nina Kirsanova said the atmosphere had changed markedly for the worse over the past week as a result of the shelling.

“Schools aren’t working, children are sitting at home. There are fewer people on the streets and everyone rushes to get things done, to get home faster,” she told Reuters.