A new generation of Al Qaeda leaders has emerged in Pakistan's tribal areas to cement control over the network's operations, American intelligence officials said.

The new leaders rose from within the organisation after the death or capture of the operatives that built Al Qaeda before the September 11, 2001, attacks, they said.

The new line up has led to surprise and dismay within United States intelligence agencies about the group's ability to rebound from an American-led offensive, The New York Times reported.

American, European and Pakistani authorities have for months been piecing together a picture of the new leadership, based in part on evidence-gathering during terrorism investigations in the past two years.

Intelligence officials also have learned new information about Al Qaeda's structure through intercepted communications between operatives in Pakistan's tribal areas, although officials said the group has a complex network of human couriers to evade electronic eavesdropping.

Investigators have also relied on interrogations of suspects and material evidence collected after a plot US and British investigators said they averted last year to destroy multiple commercial aircraft after take off from London, the newspaper said.

The investigation into the airline plot has led officials to conclude that an Egyptian paramilitary commander called Abu Ubaidah al-Masri was the Qaeda operative in Pakistan orchestrating the attack, officials said.

Masri, a veteran of the wars in Afghanistan, is believed to travel frequently over the rugged border between Pakistan and Afghanistan.

He was long thought to be in charge of militia operations in the Kunar Province of Afghanistan, but he emerged as one of Al Qaeda's senior operatives after the death of Abu Hamza Rabia, another Egyptian who was killed by a missile strike in Pakistan in 2005.

The evidence, officials said, accumulating about Masri and a handful of other Qaeda figures has led to a reassessment within the American intelligence community about the strength of the group's core in Pakistan's tribal areas, and its role in some of the most significant terrorism plots of the past two years, including the airline plot and the suicide attacks in London in July 2005 that killed 56.

Although the core leadership was weakened in the counter-terrorism campaign begun after the September 11 attacks, intelligence officials now believe it was not as crippling as once thought.

That reassessment has brought new urgency to joint Pakistani and American intelligence operations in Pakistan and strengthened officials' belief that dismantling Al Qaeda's infrastructure could disrupt nascent large-scale terrorist plots that may already be underway, the report added.

In February, the Times said, CIA Deputy Director Stephen R Kappes accompanied Vice President Dick Cheney to Islamabad to present to President Pervez Musharraf the intelligence inputs on Al Qaeda's growing abilities and to develop a strategy to strike at training camps.

US officials have told the paper in recent years that the roles of Osama bin Laden and his lieutenants in Pakistan's remote mountains have diminished with the growing prominence of the organisation's branch in Iraq and the emergence of regional terrorism networks and so-called home-grown cells.

That view, in part, led the CIA in late 2005 to disband Alec Station, the unit that for a decade was devoted to hunting bin Laden and his closest advisers and to reassign analysts within the agency's counter-terrorist centre to focus on Al Qaeda's expanding reach.

Officials believe that in contrast with the somewhat hierarchical structure of Al Qaeda in Afghanistan before September 11, the group's leadership is now more diffuse, with several planning hubs working autonomously and not reliant on constant contact with bin Laden and his deputy Ayman al-Zawahri.

Much is still not known about the backgrounds of the new Qaeda leaders. Some have adopted noms de guerre, it said.

Officials and outside analysts told the Times that they tend to be in their mid-30s and have years of battlefield experience fighting in places like Afghanistan and Chechnya.

The new cadre includes several Pakistani operatives.

Other operatives believed to be plotting internationally include Khalid Habib, a Moroccan, and Abdul Hadi al-Iraqi, a Kurd who had served in former Iraqi ruler Saddam Hussein's army, officials said.