Fighting in eastern Ukraine that began almost a year ago has resulted in the death of more than 6,000 people, in what is described as "merciless devastation of civilian lives and infrastructure" and possible crimes against humanity, the United Nations said on Monday.

"The estimated number of people killed in eastern Ukraine since April 2014 has now passed the 6,000 mark, in spite of successive ceasefires," the U.N. Human Rights Office wrote in its new report, covering the period between Dec. 1, 2014 and Feb. 15, 2015, The Associated Press reported.

Following a sharp escalation in fighting in recent weeks, more than 840 civilian and military personnel have been killed and over 3,400 wounded since mid-January, the U.N. said. Hundreds are still missing and many have been buried without their deaths recorded.

Although Russia adamantly denies any involvement in the fighting, the U.N. cites credible reports indicating otherwise.

"Credible reports indicate a continuing influx of heavy and sophisticated weaponry to armed groups in the Donetsk and Luhansk regions, as well as foreign fighters, including from the Russian Federation," the report said.

The inflow of troops and heavy weaponry from Russia was confirmed by the Organization for Security and Cooperation in Europe, according to Ivan Simonovic, the U.N. assistant secretary general, reported The New York Times.

"This has fueled the escalation of the conflict and new offensives by armed groups, undermining the potential for peace as armed groups extend their areas of control," the report said. "This has resulted in further and significant increases in civilian and military casualties."

The report details "credible allegations" of torture, arbitrary detention, and enforced disappearances committed mostly by pro-Russian separatists, but "in some instances," also by Ukrainian law enforcement agencies.

Simonovic said there has been indiscriminate shelling of densely populated civilian areas in both government and rebel-controlled areas.

Civilians often choose to remain in embattled regions "because they fear for their lives if they try to move," said U.N. rights chief Zeid Raad al-Hussein. "Many other stay to protect children, other family members, or their property."

There are now 1 million registered internally displaced civilians, according to the report.

Zeid called for both the Ukrainian government and rebel fighters to respect the Feb. 15 Minsk peace deal, warning, "Should this trend continue, this would represent a new and very deadly chapter in this conflict."