At least four people were killed and two others injured in a US drone strike in Pakistan's northwest tribal area of North Waziristan last night. The US drones fired two missiles at a house suspected of being a militants' hideout in the Tehsil Shawal area of North Waziristan, leaving at least four people inside the house killed and two others wounded, reported local Urdu TV channel (Dunya). Identities of the killed and injured are not immediately known. Located in northwest Pakistan, North Waziristan is believed to be a haven of militants who often launch cross-border attacks on the U.S.-led NATO troops stationed in Afghanistan. As North Waziristan is a mountainous area difficult for ground operations, the United States has been using unmanned aircraft to attack the militants hiding in this area though such attacks have violated the territorial rights of Pakistan. Drone strike has become one of the major factors affecting the US - Pakistan relationship. The attack came despite Pakistani demands to halt the airstrikes. Pakistan says they violate its sovereignty. Unintended casualties in the attacks have also stirred tensions.