Pro-Russian militants walk to their positions to fight against Ukrainian government troops at a checkpoint blocking the major highway which links Kharkiv, outside Slovyansk, eastern Ukraine. (AP Photo/Alexander Zemlianichenko, file)

Kiev - The United Nations has received reports of abductions and intimidation of election officials in eastern Ukraine and fears many people will flee the region due to a mounting sense of lawlessness, a UN human rights official said on Monday.

Ivan Simonovic sounded the alarm as Ukraine's interim government conceded that many voters in two eastern regions now under the control of pro-Russian separatists would not be able to take part in next Sunday's presidential election.

“We do have information on a number of presidents, of vice presidents of electoral commissions being abducted, being maltreated (in the eastern regions), with implications for a number of other members of the commissions,” Simonovic told Reuters in an interview.

“There is intimidation. So besides direct security challenges there are also technical problems concerning the holding of the elections,” said Simonovic, who is UN Assistant Secretary General for Human Rights.

Many towns in the south-east are run by self-appointed rebel mayors and policed by informal militias who have occupied administration buildings and driven out pro-Kiev officials

Kiev hopes the May 25 election will help usher in political stability after a prolonged period of turmoil that has included mass street protests, the flight of pro-Moscow president Viktor Yanukovich and Russia's annexation of Ukraine's Crimea region.

The Kiev government and its Western backers accuse Russia of fomenting separatist unrest to destabilise Ukraine and undermine the vote. Moscow, Ukraine's Soviet-era overlord, denies such accusations and accuses Kiev of riding roughshod over the rights of the mainly Russian-speaking population of eastern Ukraine.

Ukraine's Interior Minister Arsen Avakov told a news conference in Kiev on Monday the security situation in the east would severely disrupt the May 25 election in two regions.

“We clearly recognise that on the vast territory of the Donetsk and Luhansk regions there is no way to hold elections in a normal way,” he said.

The UN's Simonovic has just returned from a trip to Donetsk, an industrial city of about one million people which is now the centre of the self-declared Donetsk People's Republic.

“I felt in Donetsk fear. And it is widespread. And the fear is not only related to security challenges. It's also related to common criminality. There is a feeling of lawlessness of being unprotected and it's also accompanied by fear that there may be, rather soon, a failure of social services,” he said.

“A lot of people are preparing to leave, not only because of security but because of their social and economic prospects. It may be a big exodus and it's going to be a major challenge.”

Simonovic was involved in the drawing up of a UN report on human rights violations in Ukraine that drew accusations of political bias last week from Russia.

Moscow said the report ignored violations of human rights by the Kiev authorities.

Simonovic dismissed the charge of political bias on Monday, saying UN rights monitors dealt only with “facts”.

A Croatian, Simonovic said he saw disturbing parallels between the situation developing in eastern Ukraine and the violent breakup of Yugoslavia in the 1990s.

“It was tragic that a political solution at Dayton (the peace talks which ended the Yugoslav wars) only came after more than 100,000 victims,” he said, adding that he hoped the sides in Ukraine would avoid such a fate.