Loud explosions have been heard in the centre of Donetsk in eastern Ukraine.

A separatist leader said government forces were trying to enter the rebel-controlled city and that fighting was underway near a railway station.

An official from the rebels’ self-proclaimed Donetsk People’s Republic said at least four tanks and armoured vehicles were trying to break in.

The Ukrainian military would not comment on the reports, saying only that the general operation against the separatists was in an “active phase”.

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, especially at the airport.

A major government operation is underway around Luhansk, another main city in the east. At least one correspondent thinks it unlikely that the Ukrainian military has the capacity to launch a major offensive in Donetsk simultaneously.