The United Nations warned Friday of an "alarming deterioration" of human rights in eastern Ukraine, where an armed insurgency by pro-Russian separatists is threatening a presidential election just over a week away.
In a new report, the UN rights chief catalogued a litany of "targeted killings, torture and beatings, abductions, intimidation and some cases of sexual harassment" which she said was carried out by anti-government groups in the east.
With the May 25 vote rapidly approaching, Kiev's interim leaders are battling to keep the country from disintegrating further after Russia's disputed annexation of Crimea in March.
Government forces are pressing on with a military offensive to put down the bloody rebellion in Ukraine's industrial belt where well-armed insurgents have seized over a dozen towns and cities in just a few weeks.
The West has been pushing an OSCE peace plan to try to resolve the escalating crisis on Europe's doorstep, while threatening further sanctions if Moscow or its "proxies" disrupt the election.
'Bent on tearing Ukraine apart'
In a veiled reference to Russia, UN High Commissioner for Human Rights Navi Pillay 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".
Two of the main eastern regions, Donetsk and Lugansk, have already proclaimed sovereignty after weekend independence referendums rejected as illegitimate by Kiev and the West, raising concerns about how election will be conducted in rebel-held areas.
Pillay also voiced deep concern about harassment and intimidation of ethnic Tatars in Crimea, which was annexed by Russia in the face of international outrage.
But Moscow blasted the report as biased in favour of Kiev's leaders, who took power in February after months of pro-EU protests that led to the ouster of Kremlin-backed president Viktor Yanukovych.
"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," the foreign ministry said.
Fighting rages almost every night in the east, particularly around Slavyansk, the epicentre of the uprising, and dozens of people have been killed since Kiev launched what it called its "anti-terrorist operation" in mid-April.
The UN put the death toll from violence in the southeast, including a fierce building inferno in the port city of Odessa in early May, at a "disturbing" 127.
Ukrainian troops, backed by tanks and heavy weaponry, have set up control posts to try to seal off Slavyansk but is struggling to crush the rebellion despite superior firepower.
Pillay said there had been a "worrying" rise in abductions in the east and unlawful detention of journalists, activists, local politicians, representatives of international organisations and members of the military -- the bodies of some dumped in rivers.
She also highlighted the deteriorating risks for media in the east, where residents say confusion reigns about who is in charge.
Dozens are also still missing after months of pro-EU protests in Kiev, where over 120 people were killed in several days of bloodshed, her report said.
'Let Ukraine vote'
There are reports almost daily about attacks by armed militants on checkpoints and official buildings such as police stations and election centres, but often are impossible to confirm.
As Western alarm grows, US Secretary of State John Kerry piled the pressure on Moscow Thursday, warning of punishing new US and EU sanctions if it disrupts the election -- seen as key to preventing to country from descending into all-out war.
"Our message is really, quite simple: 'Let Ukraine vote. Let the Ukrainian people choose their future'," he said.
"Far from defending the rights of the people in the east they are seeking to speak for everybody through the barrel of the gun."
President Barack Obama has already drafted an executive order for sanctions across key sectors such as banking, energy, defence and mining.
In Kiev, visiting Polish Foreign Minister Radislaw Sikorski also said Russia "must work to ensure that election is conducted in the most legitimate manner possible".
While Moscow has rolled back on its initial vehement opposition to the vote, Foreign Minister Sergei Lavrov has questioned how it could take place against a background of bloodshed.
"When Ukrainians kill Ukrainians I believe this is as close to a civil war as you can get."
He also complained about the failure of Kiev authorities to include the separatists in the first round of "national unity" talks held under OSCE auspices on Wednesday.
Interim President Oleksandr Turchynov says he is ready to reach out to pro-Russians but that the separatists must first lay down their arms.
The Ukraine standoff has plunged relations between Moscow and the West to their lowest since the Cold War, with thousands of Russian troops still believed to be on the border.
In a further concern for Europe, Russian President Vladimir Putin has threatened to cut off gas supplies to Ukraine if it fails to pay a huge 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 $17 billion IMF aid package.