Fighting in Ukraine has escalated to levels not seen since February, amid warnings of a humanitarian catastrophe in the run-up to Christmas.

Six soldiers were killed and eight civilians injured in the flare-up in the past two days, which saw hundreds of mortar rounds, artillery shells, and rockets fired in the Donetsk and Luhansk areas in east Ukraine.

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Volker spoke of a "hellish 48 hours" on the eastern front (Photo: Christopher Bobyn)

International monitors from the OSCE, a European security body, also recorded 16,000 ceasefire violations in the period from 11 to 17 December, a 35 percent increase on the week before.

The escalation centred on the Ukraine-controlled village of Novoluhanske and on a water filtration plant near Donetsk, where dozens of people were trapped in a bomb shelter amid the risk of damage to pipes that carried poisonous chlorine gas.

"This is considered extremely dangerous. If those [pipes] were to go off in this area, which is close to where people live, it could be potentially devastating," Heather Nauert, a state department spokeswoman, said in Washington on Tuesday (19 December).

The fighting comes amid heavy snow and freezing conditions, with shortages of medicine, food, and heating fuel reported by the UN and by the Red Cross.

It also comes after Russia, on Tuesday, withdrew its delegation from the Joint Centre for Coordination and Control, a body that had been created to help keep the OSCE monitors safe.

"We note with concern a sharp deterioration in the security situation with ceasefire violations reaching levels not recorded since February this year," Ertugrul Apakan, the head of the OSCE mission, said in Kiev.

"A return to normality, wherein civilians can once again resume their everyday lives without fear of shelling and landmines is the first step to a long-term sustainable peace," he said.

The US state department spokeswoman dismissed the Russian side's claims that the Ukrainian army had started what she called the "spiralling violence".

"Russia and its proxies are the source of violence in eastern Ukraine, and the Russian government continues to perpetuate an active conflict and humanitarian crisis through its leadership and supply of military forces on the ground, as well as its direct control over proxy authorities," Nauert told press.

"The conflict in eastern Ukraine is not an organic civil war," she said.

Kurt Volker, a US diplomat tasked with handling the conflict, also warned against international complacency in a speech at the Atlantic Council, a think tank in Washington, the same day.

"A lot of people think that this has somehow turned into a sleepy, frozen conflict and it's stable and now we have … a ceasefire," he said.

"That's completely wrong. It is a crisis. This has been the most violent year, 2017, and, frankly, last night was one of the most violent nights, certainly since February, and possibly this year," he said.

He added on Twitter that the "decision for peace lies with Russia".

"Peace in eastern Ukraine can come if Russia pulls out its forces and stops support for its proxies," Volker said.

EU leaders, last week, extended the life of economic sanctions on Russia in their recognition of the Kremlin's role in the conflict.

The US and Canada have also spoken of shipping defensive weapons to Ukraine if the situation persisted.

But Pyotr Ilyichev, Russia's deputy UN ambassador, told Tass, a state news agency, that they were already doing it in secret.

He said the US was "already supplying weapons to Kiev, gaining a consent of both the State Department and the Pentagon behind the scenes, although officially the US declares that there is no final decision on this matter."

"It is clear that the pumping of Ukraine with American and Canadian means of war pushes the country's leadership, which sabotages the Minsk agreements, to new military adventures," he said, referring to an old ceasefire accord signed in the Belarusian capital.

The three and a half year-long war has claimed 10,300 lives and displaced around 1.5 million people, the UN said.