Top al Qaeda strategist killed in Pakistan By Mark Hosenball and Jibran Ahmad WASHINGTON/PESHAWAR (Reuters) - A U.S. drone strike in Pakistan killed one of al Qaeda's most powerful figures, the U.S. government said on 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. Some private analysts agreed, saying that the group that launched the September 11 attacks in the United States in 2001 now faces a crisis over its future. But even as al Qaeda's core group, now led by Ayman al-Zawahri, reels from mounting losses, its affiliates elsewhere - particularly in Yemen - have continued planning attacks on U.S. and other Western targets. The drone strikes, which have escalated in number over the last two weeks, have deeply angered Pakistan's government and contributed to unrelenting tensions between Washington and Islamabad. Pakistan summoned 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 was one of a handful of al Qaeda officials bin Laden relied on 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. Mirali is a town in Pakistan's tribal areas. Residents of the village said to be the site of Libi's death, Hesokhel, noted an unusually high number of militants gathered there after the drone strike and they kept people away. President Barack Obama has made strikes against anti-U.S. militants, particularly the killing of bin Laden, a major component of his bid for re-election in November. There have been eight such strikes since Obama and Pakistani President Asif Ali Zardari attended a NATO summit in Chicago on May 21, where they talked briefly but held no formal meeting. Three of those strikes occurred on Saturday, Sunday and Monday. It remains to be seen whether al Qaeda's core organization can recover from Libi's death. Zawahiri, the Egyptian who leads the group, is described by U.S. counter-terrorism experts as a divisive and uncharismatic figure. Some analysts say the death of one al Qaeda leader does not necessarily spell disaster for the group, arguing that the network is decentralized and offers inspiration to militants, 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 of "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 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 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, he said. He said 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. Kamran Bokhari, vice president for Middle East & South Asia Affairs at political risk analyst Stratfor, said that although Libi's death was a victory in the fight against al-Qaeda, "It's really unclear what the endgame is." "Will there ever be a time when the U.S. government can say that al-Qaeda has been sufficiently decimated?" he asked. (Additional reporting by Katharine Houreld in Islamabad and William Maclean in London. Writing by Michael Georgy and Warren Strobel; editing by Christopher Wilson)