United Nations Secretary General Ban Ki-Moon said he is "dismayed" at an attack by pro-Russian separatists that killed 13 government troops in eastern Ukraine.
Separatists used mortars, grenade launchers and automatic weapons to attack a military checkpoint Thursday near the town of Volnovakha in the Donetsk region.
The attack further raised fears of violence ahead of Sunday's presidential election, which many hope will help resolve a political crisis threatening to split Ukraine.
In a statement, Ban called on all parties to redouble their efforts to make sure that voters across the country can cast ballots in a free and fair election without fear for their safety.
The government is deploying tens of thousands of police to secure the vote. That task could be difficult in the east, where rebels have seized over a dozen cities.
Ukraine's acting Prime Minister Arseniy Yatsenyuk on Thursday accused Russia of trying to escalate the conflict and disrupt the election, which Moscow has reluctantly supported.
Russia has accused Kyiv of stepping up military operations in eastern Ukraine and failing to implement measures to end the crisis.
Meanwhile, Russia's defense ministry said 20 trains and 15 planes full of troops have been moved out of the border area with Ukraine.
Also Thursday, NATO chief Anders Fogh Rasmussen said there has been "limited" Russian troop activity near the Ukrainian border that "may suggest some of these forces are preparing to withdraw."
NATO's top military commander, U.S. General Philip Breedlove, said Thursday that "a very large" Russian force remains in the area "in a very coercive posture."