GENEVA — The United Nations said on Monday that a sharp escalation in recent fighting in eastern Ukraine had left more than 800 people dead and over 3,400 wounded, with hundreds missing and many buried without their deaths being recorded.

Ivan Simonovic, the United Nations assistant secretary general, told reporters here that more than 6,000 people had been killed since the fighting started in April — at least 842 of whom died from mid-January to the middle of last month.

The casualty figures were included in a United Nations report issued on Monday that also said an influx of troops and heavy weapons from Russia had intensified the conflict in Ukraine.

“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 United Nations report said.