DONETSK Ukraine (Reuters) - Heavy artillery fire rocked northern districts of Donetsk, the largest city of eastern Ukraine, on Saturday and a large plume of black smoke hung over the airport, a Reuters reporter said, in the biggest test in days for a fragile regional ceasefire. Donetsk airport remains in the hands of Ukrainian government forces, while the city - an industrial hub with a prewar population of about one million - is controlled by pro-Russian separatists. Earlier, the Ukrainian military said the rebels had fired on their forces overnight in the area of the airport. (Reporting By Anton Zverev, writing by Gareth Jones)