A suspected U.S. drone fired a missile on Thursday into Pakistani territory on the Afghan border, killing at least six people.
The missile hit a house in the village of Sam in South Waziristan, in an area known as a stronghold of Baitullah Mehsud, head of Pakistani Taliban militants.
NBC News reported that the blast killed six people and injured five others.
An intelligence official told Reuters that the dead included at least one "militant".
"Guests were staying there," said another intelligence agency official, using a term commonly used to refer to foreign militants.
Pakistani military spokesman were not immediately available for comment.
U.S. officials say al-Qaida and Pakistani Taliban militants operate out of safe havens in northwest Pakistan, training for an intensifying insurgency in Afghanistan that has helped make that country deadlier now than Iraq for U.S. troops.
U.S. impatience has been growing over what Washington sees as Pakistan's failure to eliminate the militant threat from their sanctuaries in remote ethnic Pashtun regions.
U.S.-operated drones, or pilotless aircraft, have stepped up strikes in Pakistan since the beginning of September, firing missiles at suspected militants 11 times and killing dozens of people, most of them militants, Pakistani security officials have said.
Two explosions reported
The second intelligence official said two missiles were fired on Thursday. A resident of Sam said by telephone two big explosions had shaken the village.
The first intelligence official said militants had cordoned off the area and were not letting anyone approach.
Cross-border strikes by U.S. forces, in particular a Sept. 3 raid by U.S. commandos on a Pakistani village, have angered Pakistan and led to calls from opposition politicians for an end to help for the U.S.-led campaign against militancy.
Pakistan rules out foreign military strikes on its territory, saying they not only violate its sovereignty but are counter-productive, increasing support for militants in a country where many people oppose support for the United States.