Suspected U.S. drone-fired missiles struck a house early on Thursday in a restive tribal region in northwest Pakistan, killing two militants, officials said.

Two intelligence officials, who spoke on condition of anonymity because they were not authorized to speak to media, said the missiles hit a house in Azam Warsak village in the South Waziristan tribal region.

The house, owned by a local tribesman, Ashraf Mahsud, was occupied by Arab militants affiliated with Al Qaeda, the officials said but did not provide more details about those killed or the airstrike itself. Mahsud, who is known to be associated with Uzbek militants operating in other parts of the region, was not at the house at the time, the officials said.

According to one of the two officials, who is based in Wana, the main town of South Waziristan, most of the Al Qaeda-linked foreign militants have left the tribal regions but some are still hiding in inaccessible pockets in the area.

South Waziristan is considered to be a sanctuary for local and Al Qaeda-linked insurgents who carry out attacks on both sides of the border. The Pakistani army has carried out a massive operation there but militants still have a presence.

U.S. drone strikes are deeply unpopular in Pakistan, where they are seen as a violation of national sovereignty and an attack that too often results in the killing and wounding of civilians. Washington has long used them to target militants in areas inaccessible to Pakistan's army.

The covert drone program has been defended by Washington as a way to target militants who threaten the United States from areas where local governments cannot or will not act against them.