An election of separatist leaders in eastern Ukraine is an obstacle to peace and the European Union will not recognise it, the EU's new foreign policy chief Federica Mogherini said.
"I consider today's 'presidential and parliamentary elections' in Donetsk and Luhansk 'People's Republics' a new obstacle on the path towards peace in Ukraine.
"The vote is illegal and illegitimate, and the European Union will not recognise it," Ms Mogherini said in a statement after pro-Russian rebels elected a separatist leadership.
Early local elections in accordance with Ukrainian law, as foreseen in the Minsk agreement, were "the legal and legitimate means of renewing the democratic mandate of the local authorities in these parts of Ukraine.
"I call on all sides to work towards such elections," she said
Russia has said it recognises the results of the controversial elections, which it said will help re-establish "normal life in the region."
"We respect the the expression of the will of the residents of southeast (Ukraine)," the foreign ministry said in a statement.
"Those elected have received a mandate to resolve the practical issues of re-establishing normal life in the region."
Earlier, Ukrainian President Petro Poroshenko called on Russia not to recognise the vote, which he described as illegitimate.
"The farce that is being conducted under the threat of tanks and guns by the two terrorist organisations in parts of Donbass is a terrible event that has nothing to do with the real will," Mr Poroshenko said in a statement.
Alexander Zakharchenko, prime minister of the self-declared Donetsk People's Republic, was on course to claim 81.37% of the vote in the presidential poll, election chief Roman Lyagin said.
His party was also set to claim 65% of the parliamentary vote.
The United States and European countries, which have imposed heavy economic sanctions on Russia, back Ukraine in condemning the polls as illegal.
No international election monitors were present for the vote and no minimum turnout was set by the organisers, reflecting the uncertainty over how many voters could turn out.
"These elections are important because they will give legitimacy to our power and give us more distance from Kiev," said Mr Lyagin.
Mr Poroshenko called the polls "pseudo-elections that terrorists and bandits want to organise on occupied territory".
The war has killed more than 4,000 people, including more than 300 in the last two weeks, since erupting in April.
A month earlier, Russian troops invaded Ukraine's southern province of Crimea, which was then annexed by Russia.
Meanwhile, the Ukrainian military has reported "intensive" movement of troops and equipment from Russia into the separatist controlled area in eastern Ukraine.
"There is intensive deployment of military equipment and personnel of the enemy from the territory of the Russian Federation onto territory temporarily controlled by insurgents," said Andriy Lysenko, spokesman for the military.