U.S. officials say a drone strike in northwestern Pakistan targeted al-Qaida's second in command, but they do not know if the militant leader was killed.

The officials, speaking to U.S. media on condition of anonymity, said the airstrike Monday in the North Waziristan tribal region targeted Abu Yahya al-Libi. They said his death, if confirmed, would be a major blow to the terror group.

Militants and residents in the area also spoke on condition of anonymity, saying they believe al-Libi was in the compound at the time of the strike.

Pakistani officials said the attack in Hassu Khel killed at least 15 people — one of several airstrikes in recent days that killed at least 27 people overall.

Pakistan's foreign ministry on Tuesday summoned Deputy U.S. Ambassador Richard Hoagland to convey “serious concern regarding drone strikes in Pakistani territory.” The ministry called the strikes “unlawful, against international law and a violation of Pakistan's sovereignty,” and a “red-line” for the country. It also noted that Pakistan's parliament had “emphatically stated” that drone strikes are “unacceptable.”

Christopher Snedden, a South Asia analyst with the Melbourne-based security consultancy Asia Calling, told VOA the United States is likely to continue the drone strikes because they are an effective way to go after militants without endangering U.S. forces.

“I expect that they will do that because it's such an efficient way of running an operation. It may be reduced if Islamabad and Washington can actually improve their relationship but there's nothing in the short term that suggests that's going to happen.”

The Libyan-born al-Libi has been running al-Qaida's day-to-day operations in Pakistan's tribal regions as well as its links to regional affiliates. He escaped prison at Bagram Air Base in Afghanistan in 2005 and appeared in a series of propaganda videos before rising to become the terror group's deputy leader last year. The U.S. government had placed a $1 million price on his head.

Relations between Washington and Islamabad have reached a new low following last year's killing of al-Qaida leader Osama bin Laden in Pakistan by U.S. special forces and the accidental killing of 24 Pakistani soldiers in a NATO air strike.

Pakistan's parliament has demanded a U.S. apology for the deadly cross-border attack last November and an end to the U.S. drone strikes. Washington refused to end the missions, saying drone strikes are a vital tool in the war against al-Qaida and the Taliban. Islamabad responded by blocking NATO supply routes into Afghanistan and the two sides have yet to reach an agreement on reopening them.

Senior U.S. defense official Peter Lavoy is set to hold talks with Pakistani officials in Islamabad this week to try and break the deadlock over the supply routes.

Pakistan's foreign ministry also lodged a formal complaint Tuesday about an incident the previous day in which police in the northwestern city of Peshawar briefly detained three U.S. diplomats after weapons were found in their vehicle.

The ministry told Deputy U.S. Ambassador Hoagland that “the carrying of unauthorized weapons by diplomats was unacceptable and contrary to both Pakistani law and accepted norms of diplomatic conduct.”