"I regret the decision of the Serbian government to reject the proposals and call on them to make a last effort to reach an agreement, for the benefit of their people," Ashton said in a statement from Brussels.
She said she believed that after several rounds of talks "all the elements for an agreement on northern Kosovo are on the table" and that this should be an agreement between the two sides.
It is not on the European Union to impose the agreement, Ashton wrote, adding that she believed in a brilliant future for both Serbia and Kosovo.
She said that regardless of how tough the negotiations were, they made it possible for people, notably in northern Kosovo, to have a better life, and voiced hope that Kosovo and Serbia would not miss the opportunity to leave the past behind and move forward into the future.