al-Qaeda has effectively declared war on the Pakistani government led by General Pervez Musharraf, and most experts believe the group's leaders including Osama bin Laden are hiding in Pakistan's northern tribal areas.

"Despite three years of our efforts to chase these guys, we have not been able to degrade their network," Riffat Hussain, who heads the department of strategic studies at Islamabad's Quaid-e-Azam university, told AFP.

"It indicates the problem is far more serious than one had anticipated. It also shows the resilience of al-Qaeda cells to re-group and revitalize, then operate and plan from these areas."

The arrest of at least 18 al-Qaeda suspects in the past three weeks represents the biggest haul so far in the nearly three-year hunt for militants since the September 11 attacks, which has already netted over 500 suspects.

Two alleged key operatives have provided a particularly chilling insight into the network's ability to plan, communicate, and coordinate -- from Pakistan -- operatives in Britain, South Africa, Southeast Asia and the US.

Tanzanian Ahmed Khalfan Ghailani, indicted by the US for his alleged role in the 1998 US embassy bombings in east Africa, and Pakistani computer genius Naeem Noor Khan, together relayed coded al-Qaeda messages around the world and hatched plots to attack as far afield as London and Johannesburg.

"Their group is responsible for al-Qaeda's external operations in the United States, Britain, Malaysia, Nepal, Indonesia and some other countries," a senior security official told AFP, asking to remain anonymous.

There could be many more operatives like Khan, as yet undetected, a Karachi-based security official said.

"Khan is just one man, carrying explosive information and moving around virtually unnoticed. We don't know how many like him are still around," the official told AFP.