"We reiterate the EU's strongest support for the holding of free, transparent and fair presidential elections," EU foreign affairs head Catherine Ashton said in a statement ahead of Sunday's vote.
"Successful elections will be a major step to de-escalate tensions and restore security and political stability," Ashton added.
Condemning what she described as "further attempts to destabilise the situation," Ashton warned that the EU would pay "particular attention to the attitude and behaviour of all parties and international partners" towards the polls.
At least seven people were killed in fighting between pro-Russian rebels and government forces outside the eastern city of Donetsk today, a day after the deaths of 18 soldiers in the heaviest loss for the Ukraine military since the conflict began.
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Ashton said the EU was "deeply saddened" by the deaths.
Leaders of the 28 EU member states are to discuss the outcome of the vote at an informal summit on Tuesday as the bloc considers its next move after imposing travel ban and asset freeze sanctions against Russian and Ukrainian figures it believes have stoked the crisis.