A missile fired from a suspected unmanned U.S. drone slammed into a car in a Pakistani tribal region close to the Afghan border Monday, killing four people, intelligence officials and residents said.

The apparent American strike was the latest of more than 50 in the region since last year aimed at killing top Al Qaeda and Taliban leaders. Last month, the head of the Pakistani Taliban was killed in one such strike.

Monday's attack took place about 1.5 miles from the town of Mir Ali in North Waziristan, killing four people, two officials and witnesses said. The officials spoke on condition of anonymity, saying they need to remain unnamed to do their job effectively.

The identities of the victims were not known.

Witnesses Ikramullah Khan and Mohammad Salim said the missile hit a vehicle with blacked-out windows -- a style associated with Taliban fighters in the region.

Pakistan protests the U.S. missile strikes as violations of its sovereignty and say they fan support for the insurgents, but Washington has shown no sign of abandoning a tactic that it says has killed several ranking militants and disrupted their operations.

Islamist militants with roots in the border region launch near-daily attacks on Pakistan's U.S.-backed government and security forces. The mountainous, lawless area is also used as a safe haven from which to stage attacks on foreign forces in Afghanistan.

Under pressure from the West, Pakistan in May launched an offensive in the northwestern Swat Valley, which had fallen largely under Taliban control. It claims to have cleared most of Swat of the militants and killed more than 1,800 of them, although sporadic militant attacks continue.

The army announced the capture last week of five top Swat Taliban commanders, and Interior Minister Rehman Malik said Sunday authorities were now closing in on Swat Taliban chief Maulana Fazlullah.

"Fazlullah is surrounded, and he cannot escape us," Malik told reporters in Islamabad.

Pakistan's army said Monday to have killed 16 suspected militants in its latest operations in Swat and neighboring Dir district. One soldier died and another was wounded, an army statement covering the previous 24-hour period added.

Military officials also said 159 alleged militants had surrendered to security forces Monday. They include six boys recruited by the Taliban to be suicide bombers, Col. Amir Khan told reporters in Piochar, a main insurgent base in Swat.

In recent weeks, the army has reported an increasing flow of insurgents voluntarily surrendering.

Because of limited access to the region, however, the information provided by the military could not independently be confirmed.