DONETSK: Artillery fire rocked east Ukraine’s rebel stronghold of Donetsk on Sunday as separatists appeared to be reinforcing their frontline and fears grew of a return to all-out fighting.

The boom of shelling rumbled on mid-morning on the edge of Donetsk, where government forces regularly exchange heavy fire with insurgent fighters, but was less intense than overnight when mortar fire was heard close to the center for around two hours, an AFP journalist reported.

It was the fiercest combat in the region since the September 5 signing of a frequently-violated cease-fire that halted full-scale confrontations across most of the conflict zone but failed to end constant bombardments at strategic hotspots.

An AFP crew saw a convoy of 20 military vehicles and 14 howitzer cannons without number plates or markings driving through the rebel town of Makiivka in the direction of the nearby frontline around Donetsk, where Ukrainian troops are struggling.

The Organization for Security and Cooperation in Europe (OSCE) voiced concern Saturday after its monitors witnessed unmarked columns of tanks and troop carriers moving through east Ukraine in territory held by pro-Russia separatists.