Al-Shabaab fighters seen in Mogadishu, Somalia in 2009. (Mohamed Sheikh Nor/Associated Press)

Top Canadian security officials say they have intercepted or intervened in a number of cases involving Canadian youths set to join the Somalia-based militant organization al-Shabaab, but in spite of their efforts many others may have joined the group.

Canadian security officials believe at least 20 Canadian youths have been recruited by al-Shabaab — and that most of those young men have come from the Greater Toronto Area.

Al-Shabaab is based in Somalia, and believed to have links to al-Qaeda.

Canadian officials claim the group has been so successful at recruiting that it is now considered to be the number 1 threat to Canada's national security.

In an interview with CBC News, Insp. Keith Finn of Canada's Integrated National Security Enforcement Team said that in spite of the successes of intercepting some youths bound for al-Shabaab training camps, there's always concern about those who've slipped through and could eventually return to Canada as trained terrorists.

"The problem is, if they're prepared to act on it, a very small number of people can cause a great deal of damage to Canadians," said Finn.

In a video obtained by CBC News, English-speaking extremists in Somalia are heard urging youths in the West to wage holy war, or jihad.

The recruits are promised power and martyrdom.

"Radicals are whispering in their ears saying, 'You will never get a job in this country. You're not wanted. You're the enemy. You can have 10 degrees, you will never get a chance in this country. So be a man, step up to the plate and join the jihad,'" said Mohamed Abdullahi Mohamed.

Abdullahi Mohamed, 36, trained as a fighter for al-Shabaab in Mogadishu but left the organization and returned to Canada in 2009. He had originally come to Canada as a teen, in 1989.

Abdullahi Mohamed now lives and works in Toronto. He says he left al-Shabaab because he never accepted its extremist views. But many young Muslim Torontonians are still joining, he said.

"They are an organization that is recruiting, effectively, young, Western Muslim youth," he said.

In March 2010, the federal government declared al-Shabaab to be a terrorist organization.

In July, al-Shabaab claimed responsibility for bombings in Uganda that killed 74 people.

Abdullahi Mohamed said he fears that unless the federal government reaches out to young Muslims there will be more young men from Canada joining al-Shabaab.

"Help us before they use us. Employ us before they employ us. The ball is in your court federal government. Wake up before the blood is soaked in the streets of Toronto, like it was in London, Stockholm and New York."