By Mark Hosenball and Jibran Ahmad

WASHINGTON/PESHAWAR, June 5 (Reuters) - A U.S. drone strike in Pakistan killed one of al Qaeda's most powerful figures, the U.S. government announced Tuesday, dealing the biggest in a series of blows to the militant group since the raid that killed founder Osama bin Laden last year.

Abu Yahya al-Libi, a veteran militant said to have been a leader of the group's operations, and who survived previous U.S. attacks, was killed in the drone strike early Monday morning on a hideout in North Waziristan in Pakistan's tribal areas, officials said.

The White House called Libi's death a "major blow" to al Qaeda, and claimed that it will be hard for the group to find someone of similar stature to replace him.

But even as al Qaeda's core group, now led by Ayman al-Zawahri, has faced mounting losses, its affiliates elsewhere - particularly in Yemen - have continued planning attacks on U.S. and other Western targets.

And the drone strikes, which have escalated in number over the last two weeks, have deeply angered Pakistan's government, contributing to unrelenting tensions between Washington and Islamabad.

Pakistan on Tuesday called in the U.S. charge d'affaires to its foreign ministry to convey "serious concerns" over the drone strikes, a ministry statement said.

For the United States, Libi had been one of al Qaeda's most dangerous figures.

Recently released letters written by bin Laden and captured during the U.S. raid in which he was killed last year show Libi to have been one of a handful of al Qaeda officials relied upon by bin Laden to argue al Qaeda's case to a worldwide audience of militants, in particular to the young.

Libi, a cleric whose real name was Mohamed Hassan Qaid, escaped from U.S. custody in Afghanistan in 2005 and on at least one previous occasion was prematurely reported to have been killed in a U.S. drone strike.

A Pakistani Taliban leader, speaking to Reuters by telephone from an undisclosed location, said Libi "had been living in the Mirali area for quite a while. Most of the people from his group were also in Mirali. When the first missile hit, they went to the house to check the damage."

"And immediately, another missile hit them at the spot. Unfortunately, Sheikh sahib (Libi) was martyred. This is a big loss, he was a great scholar. After doctor Sahib (Zawahri), he was the main al Qaeda leader," the Pakistani Taliban leader said.

U.S. President Barack Obama has made strikes against anti-U.S. militants, and particularly the killing of bin Laden, a major component of his bid for re-election in November.

Sajjan Gohel, chief executive of the Asia-Pacific Foundation security research consultancy, said Libi was one of the few remaining key figures within al Qaeda's core.

He "has also been at the center of al Qaeda's plans to reconstitute itself and try and remount a trans-national terror campaign. This is one of the reasons he was viewed as a high value target," he told Reuters by email.

Still, some analysts say the death of an al Qaeda leader does not necessarily spell disaster for the group, arguing it is decentralized and offers inspiration to militants and not just logistical support or financing.

"Even if he's killed it doesn't matter much to the organization as long as Dr Zawahri remains alive," said Imtiaz Gul, author "The Most Dangerous Place", a book about the lawless border area between Pakistan and Afghanistan.

But Jarret Brachman, a terrorism expert who consults for U.S. government agencies, said that in his view, in its recent configuration, the coherence of al Qaeda's core organization was entirely dependent on two men: Zawahri and Libi, whose activities Brachman says he had tracked closely since 2005.

Brachman said that his view is that Libi's death is a "cataclysm" for al Qaeda's core group in terms of their ability to organize and continue to spread their ideology. "There's nobody left" in the central organization if Zawahri at some point is killed or otherwise taken off the battlefield, Brachman said.

He said that al Qaeda central still had a few operatives who were capable of "blowing people up." But in terms of being able to present a coherent ideology and theology to potential followers, Libi's death was a major blow to the organization. "He was their theological pitbull," Brachman said. (Additional reporting by Katharine Houreld in ISLAMABAD and William Maclean in LONDON; Writing by Michael Georgy and Warren Strobel; Editing by Robert Birsel amd Jackie Frank)

AL QAEDA'S MOST WANTED:

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  • Osama Bin Laden

    Al-Qaeda's Saudi leader was killed in an American raid on May 1, 2011. (AP Photo, File)

  • Ayman al-Zawahri

    Ayman al-Zawahri became <a href="http://www.foxnews.com/us/2012/04/29/5-most-wanted-al-qaida-leaders/" target="_hplink">al Qaeda's new leader</a> after the death of Osama bin Laden. He is believed to be hiding in Pakistan and regularly releases propaganda videos. (AP Photo/SITE Intel Group)

  • Abu Yahia Al Libi

    Abu Yahia al Libi was al Qaeda's <a href="http://www.huffingtonpost.com/huff-wires/20120429/us-al-qaida-top-5/" target="_hplink">de facto no. 2</a> after the death of Bin Laden. He escaped a high-security U.S. prison in Bagram, Afghanistan, in 2005 and <a href="http://www.huffingtonpost.com/2012/06/05/abu-yahia-al-libi-drone-strike_n_1569772.html" target="_hplink">was killed</a> in a strike in Pakistan in June 2012. (AP)

  • Nasser al-Wahishi

    Al Wahishi was once bin Laden's <a href="http://www.huffingtonpost.com/huff-wires/20120429/us-al-qaida-top-5/" target="_hplink">aide-de-camp</a> and now commands AQAP, al Qaeda in the Arabian Peninsula . (AFP/GettyImages)

  • Ibrahim Hassan Al Asiri

    Saudi Ibrahim Hassan al Asiri is <a href="http://www.huffingtonpost.com/huff-wires/20120429/us-al-qaida-top-5/" target="_hplink">believed to be responsible </a>for building uilding the underwear bomb used to try to bring down a Detroit-bound jetliner on Christmas 2009, as well as the printer-cartridge bombs.

  • Said AlMasri

    Al Qaeda's number 3 was <a href="http://www.huffingtonpost.com/2010/05/31/al-qaeda-number-three-reported-killed_n_595561.html" target="_hplink">killed</a> in an American drone strike May 2012. (Reuters TV)

  • Fazul Abdullah Mohammed

    Mohammed was <a href="http://www.huffingtonpost.com/2011/06/11/fazul-abdullah-mohammed-dead_n_875363.html" target="_hplink">killed</a> by the Somalian army in June 2011. He led the organization in Eastern Africa. (AP)

  • Abu Musab Al-Zarqawi

    Al Qaeda's leader in Iraq was <a href="http://www.huffingtonpost.com/2010/04/19/two-top-al-qaeda-figures-_n_542653.html" target="_hplink">killed</a> in a U.S. airstrike in 2006. (AP Photo/U.S. Department of State, HO)

  • Khalid Sheikh Mohammed

    Mohammed, the <a href="http://www.huffingtonpost.com/2012/05/06/khalid-sheikh-mohammed-trial_n_1489527.html" target="_hplink">self-described mastermind</a> of the attacks of 9/11, was captured in Pakistan in 2011 and is held at Guantanamo Bay. (AP Photo/FBI)

  • Saif Al Adel

    Al Adel was Bin Laden's former <a href="http://www.huffingtonpost.com/huff-wires/20120229/ml-egypt-arrest/" target="_hplink">security advisor</a>. He is still on the run. (Getty Images)

  • Adnan El Shukrijumah

    El Shukrijumah is responsible for Al Qaeda's external operations. He <a href="http://www.huffingtonpost.com/2010/08/06/adnan-shukrijumah-new-al_n_673164.html" target="_hplink">lived in the U.S.</a> for more than 15 years. (FBI)

  • Atiyah Abd al-Rahman

    Al-Rahman was Al Qaeda's liaison for Iraq, Iran and Algeria until he <a href="http://www.huffingtonpost.com/2011/08/27/atiyah-abd-al-rahman-al-qaeda-dead_n_939009.html" target="_hplink">was killed</a> on August 22, 2011 in Pakistan. (AP Photo/National Counterterrorism Center)