Up to 40 militants have attacked an army checkpoint, killing one soldier, a security official said on Saturday. Soldiers at the checkpoint on a bridge in Wana, the main town in the South Waziristan, retaliated after coming under fire on Friday night, said the security official. "There were 30 to 40 militants who first fired rocket- propelled grenades at our post and then opened fire with AK-47 rifles which killed one of our soldiers. But we retaliated and killed six militants," a security official in the region, who asked not to be named, told media. An intelligence official said helicopter gunships also hit militant positions in the battle.