There have been more fierce clashes in eastern Ukraine, where pro-Russian separatists are trying to capture Donetsk airport.
Heavy firing could be heard near the airport - strategically important for both sides in the conflict.
A government spokesman said the rebels had broken into one of the terminals but were later pushed back. The rebels say they have seized the airport.
A truce agreed in eastern Ukraine on 5 September looks increasingly fragile.
On Friday US Secretary of State John Kerry spoke on the phone with his Russian counterpart Sergei Lavrov to "express his concern about intensifying violence in eastern Ukraine".
"Russia must use its influence with the separatists and end these attacks immediately", state department spokeswoman Jen Psaki said.
'Russian drones'
"The situation in the area of Donetsk airport remains difficult," Ukrainian military spokesman Vladyslav Seleznyov told Ukraine's Kanal 5 TV.
"The militants, using smoke bombs as a cover, broke into the ground floor of the old terminal," he said.
Ukrainian soldiers had pushed the rebels from half the building, Mr Seleznyov added.
But leaders of the self-proclaimed Donetsk People's Republic claimed that their fighters had seized the whole airport.
Eyewitnesses earlier told local media that a number of rebels had been killed during the clashes.
A correspondent for Associated Press news agency reported seeing three rebel tanks firing at the main terminal, with sniper shots ringing out.
Fighting for the airport - a vantage point for the Ukrainian army to fire on positions in rebel-held Donetsk - has intensified in recent days. Ukraine says two servicemen have been killed and another nine wounded since Thursday.
Mr Seleznyov also accused Russia of sending drones over the airport to help the rebels' reconnaissance efforts and direct fire.
Russia has denied sending arms to the rebels or soldiers to eastern Ukraine, though it says "volunteers" have crossed into Ukraine.
(BBC)
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