The U.S. special envoy for the Ukraine conflict has said 2017 was the deadliest year in the region since the outbreak of violence three years ago, and warned that hostilities are again ratcheting up.

Kurt Volker's comments on December 19 came as international monitors reported intense shelling overnight near the town of Novoluhanske, part of the eastern Ukrainian region known as the Donbas.

UN officials reported eight civilians injured and dozens of homes damaged, with winter temperatures complicating matters.

"A lot of people think that this has somehow turned into a sleepy, frozen conflict and it's stable and now we have...a cease-fire. It's a problem but it's not a crisis," Volker said in a speech at the Atlantic Council, a Washington think tank.

"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.

A State Department official later provided RFE/RL with clarification and details, saying, "It’s the worst fighting based on SMM cease-fire violations and civilian casualties since the Minsk agreements. The cumulative impact of three years of fighting has also created the worst humanitarian impact as well, according to the UN."

Volker later posted several messages to Twitter, suggesting that just before the "massive escalation" in cease-fire violations on December 18, Russia had withdrawn its officers from a coordinating body run jointly with Ukraine that is helping to implement the cease-fire.

The Organization for Security and Cooperation's (OSCE) Special Monitoring Mission to Ukraine later confirmed the Russian side's withdrawal from the Joint Center for Coordination and Control (JCCC), the OSCE said in a statement valid as of December 18 and published on December 19.

Russia's withdrawal from the body undermines the OSCE’s operations and is an attempt by the Kremlin to force Kyiv into talks with representatives of the breakaway regions in eastern Ukraine, according to Vadym Chernysh, Ukraine's minister for temporarily occupied territories and internally displaced persons.

"The presence of Russian officers in the JCCC guaranteed, among other things, the safety of the OSCE mission's staff," Chernysh said.

Volker also warned that Russia-backed forces were close to seizing a water-treatment plant in the city of Donetsk, and he called for the withdrawal of heavy weapons from the area.

The chief monitor for the OSCE mission said a sharp deterioration in the security situation had seen cease-fire violations reaching levels not recorded since February.

Chief monitor Ertugrul Apakan said on December 19 that the latest escalation showed an established trend "in which a recommitment to the cease-fire by the sides was followed by a steady increase in the level of violence, culminating in fierce fighting."

His comments came on the heels of warnings from aid agencies over the humanitarian situation in the eastern Donbas region, particularly given a December 18 attack on the government-controlled village of Novoluhanske.

More than 10,300 people have been killed, and more than 1 million displaced, since the conflict erupted in April 2014, pitting Russia-backed separatist fighters against government forces.

Volker was appointed earlier this year to try and push forward an agreement reached between Ukraine and Russia, along with France and Germany in February 2015 in the capital of Belarus, Minsk, to end the conflict. But the agreement has gone unfulfilled.

The United States and European Union have pushed Moscow to allow a United Nations peacekeeping force to be deployed in eastern Ukraine, but there are disputes over where the force would be located, and whether it would be allowed to patrol Ukraine's border with Russia.

Volker indicated that no progress had been made in negotiations with Moscow.

With reporting by Christopher Miller in Kyiv