Intense artillery fire broke out early Sunday in the separatist bastion of Donetsk in eastern Ukraine, according to AFP journalists in the area.
Firing close to the town centre began to build up at 2:00 am (2300 GMT Saturday) and could still be heard, at the same intensity, two hours later.
It was a level of military activity not seen in the region since a barely-observed ceasefire was signed in September.
However it was not immediately possible to evaluate the consequences of the artillery bombardment due to an overnight curfew imposed by the rebel authorities in Donetsk.
The Organisation for Security and Cooperation in Europe (OSCE) voiced concern Saturday after its monitors witnessed columns of tanks, howitizers and troop carriers moving through east Ukraine in territory held by pro-Russia separatists.
The OSCE report came a day after Ukraine's military said it had spotted a large column of tanks and other heavy weapons entering the country from Russia across a section of border that has fallen under control of rebel fighters.
Ukraine's military said there was bloody fighting on Saturday around Donetsk international airport, where government forces are defending a pocket of territory near the biggest rebel-held city.