Suspected U.S. drones fired four missiles at two compounds in northwestern Pakistan near the Afghan border, killing 16 alleged militants, Pakistani intelligence officials said Wednesday.

The strikes occurred just before midnight Tuesday in Bobar village in the South Waziristan tribal area, a stronghold of the Pakistani Taliban, said the officials, speaking on condition of anonymity because they were not authorized to talk to the media.

The two compounds were hit about 20 minutes apart, said the officials. It is unclear how many suspected militants were killed in each compound.

The U.S. does not acknowledge the CIA-run drone program in Pakistan publicly, but officials have said privately that the strikes have killed many senior al-Qaida and Taliban commanders.

The Obama administration has ramped up the number of drone strikes in Pakistan's rugged tribal region in the past few years. Most of the strikes have targeted al-Qaida militants or Afghan Taliban insurgents battling U.S. forces in Afghanistan.

But the attacks have also killed Pakistani Taliban fighters, who are allied with Afghan militants but have focused their attacks inside Pakistan. A U.S. drone strike killed Pakistani Taliban leader Baitullah Mehsud in 2009.

Pakistani officials have criticized the strikes as violations of the country's sovereignty, but the government is widely believed to have supported the strikes in the past and even let the drones take off from bases inside Pakistan.

Pakistani criticism has been more muted when the attacks target members of the Pakistani Taliban or al-Qaida, rather than Afghan militants with whom the government has historical ties. Many analysts believe Pakistan sees the Afghan Taliban and their allies as potential partners in Afghanistan after foreign forces withdraw.