A US drone strike killed four people in country's tribal northwest region Wednesday, a security official said, as the Western superpower steps up its assault on the militant-hit area. "A US drone fired two missiles on a militant compound in Miranshah," one security official told a foreign news agency. "We have reports that four militants were killed." Another security official confirmed the toll and said that two others were wounded. Residents in Miranshah, the main town in tribal north Waziristan province, said militants had cordoned off the attack site and would not let anyone approach. The attack triggered a fire in the compound and smoke could be seen rising from the building half a kilometre away, a resident said.