THE AIRCRAFT COLLIDED WITH A TUG DURING THE PUSH-BACK FOR TAXI AT MEMPHIS INTERNATIONAL AIRPORT. PUSH BACK HAD BEGUN WHEN THE GATE AGENT HALTED THE OPERATION TO LOAD LATE ARRIVING PASSENGERS. ACCORDING TO THE TUG DRIVER, WHEN HE APPLIED THE TUG'S BRAKES AFTER REPOSITIONING THE AIRPLANE IN THE GATE AREA, THE AIRPLANE OVER-RAN THE TUG. BOTH ENGINES HAD BEEN STARTED DURING THE PUSH BACK. THE OPERATOR HAS ISSUED A BULLETIN TO ALL FLIGHT CREWS PROHIBITING FORWARD TOWING OF THE AIRPLANE WITH ENGINES OPERATING.