"We're seeing the possibility of a tragic future unfold," he said.

Harris -- a former chief of strategic planning for the Canadian Security Intelligence Service who now directs a terrorism intelligence program for a private sector consultancy -- said he was "extremely uneasy about what their ultimate return would imply for our security."

If the returning militia veterans were naturalized citizens or permanent residents of Canada, they would also be able to easily enter the United States, he said.

Canada's National Post newspaper reported at the weekend that some naturalized Canadians have joined the al-Shabaab ("The Youth") militia in Somalia, and that others hold leadership positions within other parts of the Islamic Courts Union, or ICU -- the loose coalition of Muslim militias that now controls Mogadishu and much of the rest of the country.

It sourced the charge to a single, un-named "Somalia expert," and neither U.S. nor Canadian intelligence officials would comment for this story.

However, al-Shabaab is a source of special concern for counter-terror agencies because its leader, Aden Hashi Farah Ayro, was trained by al-Qaida in Afghanistan before the U.S. invasion, and maintains links with the group, according to the International Crisis Group, a non-profit that monitors the world's conflict zones.

One human rights specialist, Uganda-based Hassan Shire Sheikh, told the National Post that while there was little information about the role of foreigners in the ICU, he believed "there are a good number of Somalis with various Western naturalized citizenships within the rank and the file of the ICU, and (that) may warrant more systematic investigations."

Last month, according to the Voice of America, Ayro told a rally in the southern port city of Kismayo, which his forces had just seized, that foreign fighters would now be a part of the Islamic militia movement.

Other reports said that he had Arabs, Chechens and some Central Asians among his entourage.

Pakistani officials said in recent months that at least 50 Islamic militants had left that country for Somalia to link up with jihadists like Ayro.

Last week, according to one local media report, the president of the ICU, Sheikh Sharif Sheikh Ahmed, considered the more moderate of the group's two main leaders, told a meeting in Mogadishu that the Somali Diaspora should join what he called the militia's holy war against neighboring Ethiopia.

"I am appealing to all Somali communities abroad to take part in the Jihad operation against the Ethiopian troops who want to occupy our land," Ahmed said at the event, where Somalis from Sweden presented him with $7,000 raised by the diaspora community there, according to SomaliNet, an Internet news service.

The report said another official who spoke there, Fuad Mohamed Qalaf, the head of the ICU's educational department, was a naturalized Swedish citizen who had recently relocated to Somalia after running a mosque in Stockholm for 10 years. He echoed the call for Somalis living in what he called "infidel" countries to return.

Counter-terrorism agencies have long been concerned about the possible terrorist recruitment of people with European or North American citizenship or permanent residency. They can travel much more freely than recruits with Pakistani, Somali, or other "country-of-interest" nationalities.

The concern is accentuated in chaotic failed states like Somalia, where intelligence gathering and interdiction are more difficult.

Canada is home to tens of thousands of naturalized Somalis, mostly refugees who arrived after the 1991 fall of President Siad Barre, the last functioning government the country had.

Two men of Somali origin were among the 18 arrested last June in the Toronto area, after they tried to buy the ingredients for home-made bombs from undercover investigators.

Neither al-Shabaab nor the broader ICU is listed as a terrorist organization by the U.S. or Canadian governments, Harris said. In both countries designation of a group triggers a series of severe legal consequences for its members and supporters.

U.S. Assistant Secretary of State for African Affairs Jendayi Frazer told a congressional hearing earlier this year that it was hard to get a fix on a group like the ICU. "The Islamic courts are, as we have said, quite heterogeneous, and they have come together quite recently," she said.

The Somali situation highlights the difficulties for policy-makers of trying to keep up with a constantly evolving and secretive network of jihadists.

In December last year, an International Crisis Group report on the emergent jihadi militias within the ICU said Ayro's new militia group did not even have a name. U.S. counter-terrorist officials called it "The Special Group."

"From the information we have ... the jihadists" -- those interested in waging a holy war against the United States -- "are a minority even within the Islamic Courts (Union)," Frazer said.

The Canadian authorities would be keeping a close watch for any returning radicals, Harris said.

But that might not be easy.

"Terrorist operatives have been and continue to be able to move into, within, and out of Somalia with little or no visibility to international security and intelligence agencies," J. Peter Pham of James Madison University told a recent congressional hearing.

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