Four people have been killed in US drone attack in South Waziristan on Friday. Sources said US drones once again violated Pakistani airspace and fired two missiles in at a building in tehsil Laddah in South Waiziristan killing four and injured several people. Suspected US missile strike Friday killed at least three foreign militants in northwest Pakistan, in a stronghold of an influential local Taliban commander, a senior security official said. The strike was the second in as many days in the South Waziristan tribal area on the border with Afghanistan, where hundreds of extremists sought refuge after the US-led invasion in late 2001 that drove the Taliban regime from Kabul. "Two missiles were fired on Taliban offices near Ladha village in South Waziristan, killing three foreign militants and wounding two others," a senior security official said. The official, who said the death toll could rise, had no immediate details on the nationalities of those killed. Ladha is the stronghold of Baitullah Mehsud, who leads the country's umbrella Taliban organisation, Tehreek-e-Taliban Pakistan (TTP). The shadowy Mehsud was accused by the government of former president Pervez Musharraf and US officials of masterminding the assassination of former premier Benazir Bhutto at a campaign rally in Rawalpindi in December 2007. Reports surfaced in October that Mehsud was seriously ill. His whereabouts are not currently known. The offices destroyed in Friday's strike were once a girls' school that was taken over several months ago by fighters loyal to Mehsud, the security official said.