Rebel forces in eastern Ukraine are conducting an offensive to capture the government-held airport in Donetsk, officials say.
Pro-Russian rebels have tried several times in recent weeks to take the airport, which lies to the north-west of the city, despite an official truce.
The Ukrainian military said the rebels were moving on “a broad front”.
However a spokesman denied claims they had taken a large part of the airport and insisted it was not surrounded.
Both sides have accused each other of violations since the ceasefire was called on 5 September.
Late on Thursday, a Swiss employee of the International Committee of the Red Cross was killed by shelling in the centre of Donetsk.
A spokesman for the aid organisation told Reuters news agency that the shell had landed near its office.
On Wednesday, four people were killed by a shell which landed on a school in Donetsk, and six died when a minibus was hit.
A spokesman for what the Ukrainian government calls its anti-terrorist operation said Ukrainian forces repelled four attacks on the airport on Wednesday evening.
A T-64 tank was destroyed and seven rebels were killed, Vladyslav Seleznyov told Kanal 5 TV.
The rebels used tanks, multiple-launch rocket systems, artillery and mortars, he said, resuming their attacks on Thursday morning with small-arms fire.
The airport is strategically important, lying just outside Donetsk, the largest city held by the rebels. Its capture by the separatists would help them to resupply.
Government forces have been using it to shell separatist positions inside the city.
A reporter for Associated Press in Donetsk said on Wednesday there were indications the government may already have lost control of the airport.