Pakistani intelligence officials say a U.S. drone strike has killed at least 21 militants in the country's North Waziristan tribal region along the Afghan border.

The officials say the unmanned aircraft fired two missiles into a house near the town of Miran Shah Wednesday, targeting members of the Haqqani network, a militant group affiliated with the Taliban and al-Qaida.

The dead included local Afghans as well as some Arabs and Uzbek nationals.

Initial reports said five militants were killed in the attack but officials said the toll rose to 21 after more bodies were dug out of the debris of the house.

U.S. officials do not publicly acknowledge the use of such strikes inside Pakistan but privately have confirmed their existence to various news outlets. Pakistan has condemned the attacks as a violation of its sovereignty, but the drone strikes are believed to be carried out with the help of Pakistani intelligence.

Last week, Pakistani President Asif Ali Zardari called on the United States to agree upon “clear terms of engagement” in the fight against Islamist militants operating in the country. Pakistan is seen as a key ally of the United States, but relations have been tense since the covert U.S. raid that killed Osama bin Laden in the Pakistani city of Abbottabad on May 2.