WASHINGTON -- The risk of a terrorist attack by radical Islamic extremists has grown in Canada and elsewhere because the United States and its Western allies failed to crush al-Qaeda completely during the war in Afghanistan, says Richard Clarke, the former top White House counterterrorism adviser whose dire warnings went unheeded before Sept. 11, 2001.

"We've had more major terrorist attacks by al-Qaeda and its affiliated groups in the 30 months since 9/11, than we had in the 30 months before 9/11 . . . a lot more," Mr. Clarke said yesterday in an interview with The Globe and Mail.

He called it the grim consequence of Washington's failure to completely destroy al-Qaeda in Afghanistan, where more than 2,000 Canadian troops are stationed to defend the pro-U.S. government.

Mr. Clarke, who served in President George W. Bush's administration but is now a fierce public critic, said a greater, more sustained effort by coalition troops in Afghanistan in the fall of 2001 might have crushed al-Qaeda, rather than allow many of its leaders to escape.

The terrorist network has since dispersed, becoming an even more dangerous, loose coalition of scattered cells and affiliates taking aim at the United States and its allies, he said. "Canada is as likely to be hit as anyplace else," he said.

Mr. Clarke noted that al-Qaeda may be "waiting to do a spectacular or a series of spectacular [attacks], and we do know that they take a long time to prepare these things."

The result, he said, is a "hydra-headed series of autonomous organizations which will morph over time into groups that we don't really know, and it will be more difficult for us to find [them]."

The present threat of loosely affiliated groups with cells around the world means they are "more difficult to detect and break up in advance, we don't know who the leaders are, we don't know who the foot soldiers are. It's harder to monitor that many groups, it's harder to prevent things." He also warned that "al-Qaeda's leadership had two years when it was out there free to regroup . . . to send out new instructions and perhaps to disperse funds."

Only a week before hijacked airliners slammed into the World Trade Center in New York and the Pentagon in Washington, Mr. Clarke wrote to top U.S. intelligence chiefs to stir them to action by asking how they would feel if a terrorist attack killed hundreds of Americans.

Mr. Clarke quit the White House a year ago after serving three presidents as a top counterterrorism adviser.

His new book, Against all Enemies, has shaken Mr. Bush's administration in recent weeks, with its accusation that it ignored warnings in early 2001 that al-Qaeda posed an imminent threat to Americans. He also said that the White House further weakened America's defences against al-Qaeda by diverting resources to the war against Iraq.

He said his worst nightmare is that al-Qaeda, or one of its affiliates, will deliver a radiological attack, the so-called dirty bomb, although he also called that threat "the least likely."

"There are so many vulnerable systems they could attack: railroads, . . . chemical plants, sports stadiums, shopping malls," he said.

Mr. Clarke said the United States and its allies should redouble their efforts in Afghanistan to wipe out what remains of the old al-Qaeda leadership and support a civilian government.

"The best thing they could do at this point would be to increase resources in Afghanistan -- intelligence resources, military resources and economic development resources."

He also had high praise for Canada's counterterrorism police and intelligence agencies, saying they have helped prevent attacks.

After a Canadian-based al-Qaeda cell failed in its plot to blow up Los Angeles airport in 1999, both the Canadian Security and Intelligence Service and the RCMP were "bending over backwards to be helpful," Mr. Clarke said.

"They were obviously embarrassed that there was this cell living there and they had not paid very much attention to it, nor informed us. I think they were afraid that we would overreact in some way."

The plot ended when Ahmed Ressam was stopped by an alert U.S. Customs official as he tried to enter the United States on a genuine, but falsely obtained, Canadian passport.