The United States and France on Friday warned Russia of new sanctions if Moscow keeps up its "provocative and destabilising behaviour" in Ukraine, just over a week before the ex-Soviet republic is due to hold a presidential election. The warning came as the United Nations sounded the alarm on deteriorating human rights in eastern Ukraine, citing a litany of abuses which it said was carried out by pro-Russian separatists. 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". In a phone call, US President Barack Obama and his French counterpart Francois Hollande "underscored that Russia will face significant additional costs if it continues its provocative and destabilising behaviour", the White House said in a statement. Obama has already drafted an executive order for sanctions across key sectors in Russia such as banking, energy, defence and mining. With the May 25 election fast approaching, Kiev's interim leaders are struggling to keep the country from disintegrating further after Moscow's disputed annexation of Crimea in March. Government forces are finding it an uphill battle to crush the bloody rebellion in Ukraine's industrial belt where insurgents have seized over a dozen towns and cities in just a few weeks. In a new report, the UN rights chief catalogued a series of "targeted killings, torture and beatings, abductions, intimidation and some cases of sexual harassment", which she blamed on the anti-government groups in the east. 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. - 'Old wounds reopened' - Two of the main eastern regions, Donetsk and Lugansk, have already proclaimed sovereignty after weekend independence referendums rejected as illegal by Kiev and the West, raising concerns about how elections will be conducted in rebel-held areas. Pillay also voiced deep concern about harassment and intimidation of ethnic Tatars in the Crimean peninsula, which was annexed by Russia in the face of international outrage. Separately, US Secretary of State John Kerry also denounced fresh rights abuses of Crimea's Tatars, saying: "Russia's occupation and illegal attempt to annex Crimea has reopened old wounds." "The list of human rights abuses committed today in Crimea is long and grows longer with each passing week," he said in a statement. "Murder, beatings and the kidnapping of Crimean Tatars and others have become standard fare." But Moscow blasted the UN 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 they are struggling to crush the rebellion despite superior firepower. In Donetsk on Friday, an AFP correspondent said masked gunmen had seized the National Guard headquarters in the city without meeting any resistance, and that the troops had fled. 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. 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' - Kerry on Thursday urged Moscow against disrupting the election -- seen as key to preventing the 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, accusing the rebels of "seeking to speak for everybody through the barrel of the gun". While Moscow has rolled back on its vehement opposition to the vote, Foreign Minister Sergei Lavrov has questioned how it could take place against a backdrop 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. And a second round of talks is due to take place in the eastern city of Kharkiv, but again no rebels are invited. The Ukraine standoff has plunged relations between Moscow and the West to their lowest since the Cold War. In 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.