MILAN, Dec 11 (Reuters) - A four-storey building collapsed in the Sicilian town of Ravanusa following a gas explosion and rescuers were searching for 12 missing residents, including three children, ANSA news agency reported. ANSA said the building was on fire after a natural gas pipe exploded in the town of 11,000 near Agrigento, the southwestern Sicilian city famous for its Greek temples. Firemen have been able to contain the fire and have begun digging in the rubble for the missing, including a young couple expecting a baby. The explosion damaged at least another two buildings in the centre of town, ANSA said.