Elections held by pro-Russian rebels in eastern Ukraine threaten to close the "window of opportunity" for a peaceful solution to the bloody crisis, EU foreign affairs head Federica Mogherini said Tuesday.

"The main risk I see now is that we close this window of opportunity for internal dialogue and for dialogue with Russia" on implementing a peace plan all sides agreed on in September, Mogherini told a press conference with NATO chief Jens Stoltenberg.

"They risk to put in serious danger the path of dialogue and peace" set down in the Minsk accords, she said, repeating that the polls held by separatists were illegal and would not be recognised.

"The main topic today should be how to make sure that we find a solution to the conflict," Mogherini said when asked if the polls justified further European Union sanctions against Russia for its role in a conflict which has so far cost some 4,000 lives.

The EU has kept the issue constantly under review and sanctions, as had been made clear repeatedly, could be changed depending on developments on the ground, she said.

For his part, Stoltenberg said the "so-called elections ... run directly counter" to the ceasefire and peace protocols agreed in the Belarus capital Minsk.

Russia backed the accords but then said it would recognise the rebel elections, threatening to deepen divisions on the ground in eastern Ukraine.

While the fighting has eased since the Minsk agreements, there have been major breaches on all sides, with the death toll steadily mounting.

Stoltenberg said Russia continued to arm, train and support the rebels, with special forces still inside Ukraine itself, while also moving troops along the border.

He repeated calls on Russia to use its influence with the rebels to support and implement the Minsk accords.

Ukraine President Petro Poroshenko meanwhile said Kiev remained committed to the Minsk agreements but "other participants" were not living up to their commitments.