Pakistani officials say a U.S. drone strike has killed four suspected militants in the country's northwest near the Afghan border.

Intelligence sources say the missiles hit a vehicle driven by rebels Thursday in the North Waziristan tribal region.

This marks the second drone strike in the region this week. Late Tuesday, U.S. missiles killed four suspected militants after hitting a compound near Miran Shah, the main town in North Waziristan.

U.S. officials confirmed that attack, the first of its kind since NATO helicopters mistakenly killed 24 Pakistani soldiers along the border last November.

Elsewhere in the tribal areas, officials say militants ambushed Pakistani troops on patrol near Sararogha in South Waziristan Thursday, killing four soldiers. The troops were conducting a search operation.

The November NATO attack that killed two dozen Pakistan troops worsened already strained U.S.-Pakistan relations and prompted Pakistan to block supply routes for NATO troops in Afghanistan.

There had been no drone strikes since then, but U.S. officials have denied that the recent drop-off in attacks was deliberately aimed at mending frayed relations.

Drone strikes in Pakistan are credited with killing dozens of of al-Qaida operatives and hundreds of low-ranking fighters since 2004. But they have infuriated many Pakistanis and complicated relations between Washington and Islamabad.