The United Nations has warned of an "alarming deterioration" of human rights in eastern Ukraine, blaming pro-Russian separatists, and prompting a swift denouncement from Moscow.

In a new report published on Friday, UN High Commissioner for Human Rights Navi Pillay catalogued a number of "targeted killings, torture and beatings, abductions, intimidation and some cases of sexual harassment".

Pillay also voiced concern about "serious problems" of harassment and persecution of the Tatar community in Crimea, which was annexed by Russia in March amid international outrage.

Pillay also called on those with influence on the armed groups in the east "to do their utmost to rein in these men who seem bent on tearing the country apart".

Russia slammed the report, accusing the UN of double standards.

"The complete lack of objectivity, blatant discrepancies and double standards leave no doubts that (the report's) authors were performing a political put-up job aimed at clearing the name of the self-declared authorities in Kiev," said the Russian foreign ministry in a statement.

May elections

With the May 25 vote rapidly approaching, Kiev's interim leaders are battling to keep Ukraine from disintegrating.

Government forces are pressing on with a military offensive to extinguish a bloody rebellion in Ukraine's industrial belt where well-armed pro-Russians have already seized more than a dozen towns and cities in just a few weeks.

The US and the European Union have been pushing an Organisation for Security and Cooperation in Europe (OSCE) peace plan to try to resolve the escalating crisis, while threatening further sanctions if Moscow or its "proxies" disrupt the election.

In another concern for Europe, Russian President Vladimir Putin has threatened to cut off gas supplies to Ukraine if it fails to pay a bill by early June.

Nearly 15 percent of all gas consumed in Europe is delivered from Russia via Ukraine, which is facing further economic gloom despite a $17bn International Monetary Fund aid package.

Ukraine has said it could pay around $4bn by the end of May but was demanding a lower rate for supplies, Reuters has reported.