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.
At least seven people were killed in clashes in the port of Mariupol on Friday, officials said.