ISLAMABAD, Pakistan - A U.S. drone strike in northwest Pakistan has killed al-Qaida's second-in-command, officials from both countries confirmed Tuesday, the most significant victory so far in the controversial bombing campaign and the biggest setback to the terror network since the death of Osama bin Laden.

Abu Yahya al-Libi was considered a media-savvy, charismatic leader with religious credentials who was helping preside over the transformation of a secretive group based in Pakistan and Afghanistan into a global movement aimed at winning converts - and potential attackers - from Somalia to the Philippines.

This was not the first time the U.S. had al-Libi in its sights: He was originally captured a decade ago and held by American forces at the Bagram Air Field in Afghanistan until he escaped in 2005 in an embarrassing security breach. Soon thereafter, he began appearing in videos in which he talked about the lessons he learned while watching his captors, whom he described as cowardly, lost and alienated.

White House spokesman Jay Carney called al-Libi's death a "major blow" to the group. Carney described al-Libi as an operational leader and a "general manager" of al-Qaida. He said al-Libi had a range of experience that will be hard for al-Qaida to replicate and brings the terror network closer to its ultimate demise than ever before.

"His death is part of the degradation that has been taking place to core al-Qaida during the past several years and that degradation has depleted the ranks to such an extent that there's no clear successor," Carney said.

A U.S. official familiar with the case, who confirmed that al-Libi was killed in a drone strike, said no one left in al-Qaida comes close to replacing the expertise the group has just lost. The official spoke on condition of anonymity to discuss intelligence matters.

The Libyan-born al-Libi, who was thought to be in his late 40s, was killed Monday morning in a village in northwestern Pakistan, a tribal area bordering Afghanistan that is home to many al-Qaida and Taliban members and their support networks.

A Pakistani intelligence official said late Tuesday that al-Libi was dead but declined to say how authorities knew this or whether they had seen his body. He spoke on condition of anonymity because of the classified nature of the drone program.

Al-Libi, who was considered a hero in militant circles because of his escape from the American military prison, was elevated to al-Qaida's No. 2 spot when Ayman al-Zawahri replaced bin Laden. As al-Qaida's de facto general manager, he was responsible for running the group's day-to-day operations in Pakistan's tribal areas and managed outreach to al-Qaida's regional affiliates.

Al-Libi was influential and popular within al-Qaida because of his "scholarly credentials, street cred from having escaped from Bagram, charisma and his easygoing, tribal speaking style," said Jarret Brachman, a counterterrorism expert who has studied al-Libi for the past seven years.

"People may have revered Zawahri, but they loved Abu Yahya," said Brachman.

Al-Libi was the latest in the dozen-plus senior commanders removed in the clandestine U.S. war against al-Qaida since Navy SEALs killed bin Laden in a raid on May 2, 2011 on his compound in the Pakistani military town of Abbottabad - nearly a decade after 9/11.

Perhaps the most well-known al-Qaida figure killed in a drone strike before al-Libi was Anwar al-Awlaki, a prominent member of the Yemen al-Qaida offshoot who died last September.

Al-Libi's death will likely fuel arguments in favor of the U.S. drone campaign despite Pakistani objections. Coming in an election year, it may also boost the tough-on-terrorists image President Barack Obama has tried to cultivate.

"The killing of al-Libi demonstrates the increasing proficiency and skill - plus good intelligence - at work in the decade-long American war to crush al-Qaida. It makes Barack Obama the counter-terrorist in chief in leading that war," said Aaron David Miller, a former adviser to six U.S. secretaries of state and currently a senior scholar at the Wilson Center.

The use of drones has skyrocketed under the Obama administration but has dropped off recently in Pakistan, which views the program as a violation of its sovereignty. Among the Pakistani public, the drone campaign is vilified because of its perceived civilian casualties, an allegation disputed by the U.S.

An on-the-ground investigation by The Associated Press this year found that the drone strikes were killing far fewer civilians than many Pakistanis are led to believe, and that a significant majority of the dead were combatants.

Pakistan's foreign ministry spokesman Moazzam Ahmad Khan said Islamabad had not been notified about the killing of al-Libi, and declined further comment.

Even while speculation was swirling as to whether al-Libi was alive or dead, Pakistan called Deputy U.S. Ambassador Richard Hoagland to the Foreign Ministry on Tuesday to protest the drone strikes.

Members of the Pakistani government and military have supported such strikes in the past, but that cooperation has come under strain as the relationship between the two countries has deteriorated. Last November, American airstrikes killed 24 Pakistani soldiers along the border with Afghanistan.

Pakistan responded by cutting off supply lines to NATO and American forces in Afghanistan and demanding that the U.S. end the drone attacks on its territory.

But the U.S. shows no such inclination. The White House maintains a list of terrorist targets to be killed or captured, compiled by the military and the CIA and ultimately approved by the president.

A recent uptick in drone strikes in the tribal areas indicates the U.S. was tracking al-Libi or had some idea that a top al-Qaida official was in the area. U.S. drones have struck seven times in recent weeks after a relative lull earlier this year.

Pakistani and Taliban officials said al-Libi was wounded in the days leading up to Monday's drone strike, although there were conflicting accounts as to where and when.

Evan Kohlmann, a senior partner a Flashpoint Global Partners which tracks radical Muslim propaganda, said the U.S. could have tracked al-Libi the same way it tracked bin Laden, through the use of couriers used to carry messages or in the case of al-Libi, carrying the video recordings that were his calling card.

Dozier reported from Washington. Rasool Dawar in Peshawar, Ishtiaq Mahsud in Dera Ismail Khan, Munir Ahmed in Islamabad, Sebastian Abbot in Kabul, Afghanistan, and Steven R. Hurst in Washington contributed to this report.