Donetsk: The main rebel leader in Ukraine's largest separatist region won a crushing victory in a controversial election on Sunday, according to an exit poll conducted by the insurgents.

Alexander Zakharchenko, prime minister of the self- declared Donetsk People's Republic, was on course to claim 81.37 per cent of the vote in the presidential poll, rebel election chief Roman Lyagin said. His party was also set to claim 65 per cent of the parliamentary vote.

Kiev has branded the Sunday's poll a "farce", which also threatens to deepen an international crisis over the conflict.

The election -- along with another in the neighbouring rebel Lugansk region -- has been backed by Russia and billed as a way of bringing a degree of legitimacy to the rebel control over two main industrial cities seized in an armed uprising.

However, the polls -- rejected outright by the West -- have worsened an international standoff over the conflict and further undercut the teetering month-long truce between Ukraine's government and the heavily armed separatists.

Ukrainian President Petro Poroshenko blasted the ballot as a "farce that is being conducted under the threat of tanks and guns" and warned Moscow not to follow through on its intention to recognise the result.

The run-up to the vote saw a spate of shelling by rebels of government positions across the conflict zone, where according to UN figures more than 4,000 people have died since fighting started around seven months ago.

Ukrainian authorities announced Sunday the deaths of three soldiers and seven more wounded, adding to Saturday's toll of seven dead and at least six wounded.

Fuelling concern of fresh fighting, Kiev's military also claimed it had detected "intensive" movement of troops and equipment from Russian territory.

The reported deployments, which would constitute a major escalation of Russian involvement, could not be verified.

AFP journalists in rebel-controlled Donetsk saw a column of about 20 trucks, some carrying heavy anti-aircraft guns, heading in the direction of the government-held airport, although there was a notable drop in fighting during the polling.

The rebels -- who deny being helped by Russia, but boast an arsenal that includes anti-aircraft missiles, tanks and heavy artillery -- have threatened to expand their offensive to the Azoz Sea port city of Mariupol.

As voting went on the Security Service of Ukraine said it was opening a criminal investigation into the separatist election, which it called "a power grab".

But residents of rebel-held areas spoke of their hatred for the government in Kiev and their desire for the war to end.

With AFP inputs