Heavy fighting has been reported in the centre of Donetsk in eastern Ukraine.
Four people are said to have been killed in clashes near the railway station and close to the airport.
Residents have been seen fleeing the city.
Separatists claimed government forces were trying to retake control of Donetsk, one of two major cities still in rebel hands.
The government in Kyiv denied sending the army in, saying small autonomous pro-Ukrainian groups were fighting the rebels.
President Poroshenko has vowed to retake Donetsk, which is at the heart of the rebel uprising against the Kyiv-based authorities.
Flare-ups in and around the city happen regularly.
By early afternoon on Monday the authorities were saying the situation had calmed down, although there were still reports of shelling.