The top hierarchy of al-Qaeda has taken such a hit from US missile strikes that Osama bin Laden and his deputy have had to replace people in the terrorist organisation with men they have never met, according to Western intelligence sources.

A dozen of al-Qaeda’s “senior management” have been killed by Predator drone attacks, which have been so effective in locating their targets that the militant group has been forced to move from traditional outdoor training camps to classroom-style facilities that are hidden from view.

After the success of the new weapons, which are unmanned and operate by remote control from 15,000 feet, the United States is to step up its drone attacks. On January 1 Hellfire missiles, operated from an air force base in Nevada, hit targets in the South Waziristan region of Pakistan, close to Afghanistan, and yesterday two missiles slammed into the stronghold where Baitullah Mehsud, the Pakistani Taleban leader, is believed to live.

The killings have had a huge impact on the structure, organisation and effectiveness of al-Qaeda, limiting the capacity for commanders to liaise with each other, further separating the top command from the lower ranks and introducing a high degree of uncertainty and a constant awareness of the likelihood of death lurking in the skies.

Bin Laden, al-Qaeda’s figurehead leader and Ayman al-Zawahiri, his Egyptian deputy, have had to rely on the loyalty of their associates to stay alive and remain hidden from the American surveillance networks.

Predators, armed with Hellfire missiles and precision-guided penetration bombs, have already succeeded in targeting two individuals believed to have ranked number three in the al-Qaeda chain of command: Hamza Rabia and Abu Laith al-Libi. They have also killed Mohammed Atef, reputedly the chief of military operations, and several of the group’s most experienced explosives and biological weapons specialists.

One of the consequences of the Predator attacks has been that al-Qaeda has had to give up its traditional terrorist training camps. Sending recruits out into the open to receive military-style jihadist instruction in combat and bomb-making has become too risky. “As soon as they are spotted, the Americans attack with Predators,” a counter-terrorist source said. Now terrorist training in the tribal regions in Pakistan is carried out “in the classroom”, less visible from the air and making it more difficult for the Americans to monitor the scale of the recruiting.

Communications between the top echelon and operatives is now restricted to human couriers. Mobile and satellite phones are never used by the core leaders because they know that American signals intelligence will be able to pinpoint individuals as soon as the devices are switched on.

Since the Americans acquired missile-armed Predators and the newer model, called Reaper, the CIA and Pentagon have focused on killing terrorist targets rather than monitoring and tracking the activities of suspected al-Qaeda figures. The killing option has led to an increasingly successful record.

Despite a number of attacks that led to civilian deaths, in more recent Predator missions – particularly over the past four months – the intelligence has been more accurate. In one mission in November a Predator strike on a compound in the village of Ali Khel in North Waziristan killed two of the most senior al-Qaeda operatives, Abu Zubair al-Masri, an Egyptian explosives expert, and Rashid Rauf, the British Pakistani who is alleged to have been linked to the Heathrow bomb plot of August 2006.

There were claims that Rauf was not in the compound at the time, but counter-terror officials firmly believe that he was there and that he died.

The killing of al-Libi, reputed to be a number three in the al-Qaeda hierarchy, in January last year was one of the biggest blows for bin Laden and al-Zawahiri. He was head of the Libyan fighting group of al-Qaeda and was regarded as an important director. He was also a charismatic, respected religious figure and operational planner who could smooth the way for al-Qaeda in the tribal areas whenever there were confrontations between the terrorist leaders and their Pakistani hosts over the constant threat posed by the American Predators.

Another serious loss to al-Qaeda was that of Abu Abeda al-Masri, the head of external relations who died of natural causes after becoming ill with hepatitis. He was a significant loss in terms of the threat to the UK because his role was to train Britons.

Another key Predator victim was Abu Suleiman al-Jusayi (or al-Jazairi), an Algerian who was an al-Qaeda trainer and explosives specialist. He had been involved in a series of European terrorist networks. He was killed in the Bajaur tribal district of Pakistan in June.

One of the most sought-after American targets was Abu Kabbah al-Masri, al-Qaeda’s most experienced biological weapons scientist. He was engaged in the chemical and biological trials that were uncovered in Afghanistan in 2001. He was known to be continuing his experiments in the tribal regions of Pakistan. He was tracked by the Americans and killed by a Hellfire missile in the second half of last year. Midhat Mursi al-Sayid Umar, another poisons expert, is also believed to have been killed by the Americans in a Predator attack.

The only al-Qaeda commander to have been killed by other means in the past 12 months was Abu Ghadiyah, who was in charge of the production line of suicide bombers from Syria into Iraq. He died during a controversial US commando helicopter raid across the border from Iraq in October.

Aerial assault

— Armed predator unmanned aerial vehicles (UAVs) have been in use since 1999

— The aircraft is controlled from the ground using satellite systems and onboard cameras

— The MQ9 craft, which is used in Afghanistan, is 11m long, has a 20m wing span and a cruise speed of up to 230mph. Each can carry four Hellfire missiles and two bombs

— Three systems were bought by the RAF last year for £500m

Sources: Jane’s Information, US Airforce, RAF, Times archives

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