OTTAWA - Canada's stretched spy agency can vet a mere fraction of immigrants and refugees coming from the fertile terrorist breeding grounds of Pakistan and Afghanistan, says a top CSIS official.

Deputy director Jack Hooper told the Senate defence and security committee today that a lack of resources has meant 90% of the estimated 20,000 immigrants from the region have entered Canada in the last five years without proper screening.

But Hooper said home-grown terrorists are fast becoming the greatest threat to Canada's national security. Second and third-generation Canadians are becoming radicalized here "including white Anglo-Saxon Protestant converts" and are eluding authorities because they blend in with mainstream society.

"I would say that the home-grown terrorist has achieved a condition as a phenomenon that would put it on par with what is coming at us from the outside. In terms of their relative ranking, they're on equal ground as far as we're concerned," he said.

The primary target for home-grown terrorists is Canada, Hooper said.