Pro-Russian separatists in Ukraine’s two eastern regions are holding “self-rule” referendums – a move condemned by the Ukrainian government and the West.
Self-proclaimed leaders in the Donetsk and Luhansk regions are going ahead with the vote despite Russian President Vladimir Putin’s call to postpone it.
Ukraine says the vote could result in the “self-destruction” of the regions.
Pro-Russian gunmen occupying offices in a number of towns have been involved in heavy clashes with Ukrainian troops.
Reports say there was a fierce fighting overnight on the outskirts of the rebel-held city of Sloviansk, which remains sealed by government troops conducting what the government in Kiev describes as an “anti-terror” operation.
In the port of Mariupol, also in the Donetsk region, at least seven people were killed and 39 injured in clashes between the two sides on Friday, according to official figures.
‘Total collapse’
Referendum organisers said earlier this week that most of the polling stations were being controlled by pro-Russian activists and would be ready for voting..
Millions of ballot papers have been prepared.
They contain only one question in both Ukrainian and Russian: “Do you support the act of state self-rule of the Donetsk People’s Republic/Luhansk People’s Republic?”
The organisers have suggested they intend to hold a second round later in the month – this time on joining Russia. They also say they will be boycotting Ukraine’s presidential elections on 25 May.
There are no independent or international observers involved in Sunday’s vote, and all is in the hands of the organisers, the BBC’s Sarah Rainsford in Donetsk reports.
On Saturday, Ukraine’s interim President Olexandr Turchynov admitted many in eastern Ukraine supported the pro-Russian militants, but warned that the referendums were “a step towards the abyss”.
“Those who advocate self-determination do not understand that this will mean the total collapse of the economy, of social programmes, and of life in general for the majority of people in these regions,” he said.
The EU and US have also condemned the referendums, amid fears that Ukraine could be sliding to civil war.
In Moscow, President Putin earlier called for a postponement of the vote to create the conditions necessary for dialogue.
Last month, Russia annexed Ukraine’s southern autonomous republic of Crimea, and there are fears that Moscow could invade mainland Ukraine to support the separatists.
However, the Kremlin has said it has no plans to cross the border.
Russia is estimated to have some 40,000 troops near the border with Ukraine, though Moscow says they have been pulled back.
Nato says it has seen no sign of any withdrawal.
On Saturday, French President Francois Hollande and German Chancellor Angela Merkel warned Russia of further sanctions if Ukraine’s presidential election failed to go ahead.
“If there is not an internationally recognised presidential election, that would lead unavoidably to a further destabilisation of the country,” the two leaders said in a statement.
Correspondents take this to mean economic sanctions against Russia already authorised by European leaders in March.
Both the EU and US have already imposed sanctions targeting officials and companies linked to President Putin.