TORONTO – A perimeter security concept for Canada and the United States, previously discussed in only the vaguest terms, has gained new momentum after last week's terrorist attacks.

The idea, nicknamed "Fortress North America," would aim to increase defenses against terrorism while preserving free movement along the world's longest undefended border, which is a conduit for trade worth $1.4 billion a day.

The perimeter concept would make it harder for those with terrorist links to enter either country from the outside, but retain the relatively unhindered traffic across the 6,400-km frontier.

U.S. Ambassador Paul Cellucci has openly backed the idea in recent days, saying Canada and the United States should coordinate immigration and refugee laws to make it harder for criminals to gain entry to either country.

That would allow "law-abiding citizens and the free flow of commerce" across the border.

"If we had policies of immigration and refugee status that were more common, we could establish this perimeter to protect the United States and Canada and I think that's where we should be heading," Cellucci said.

The issue is sensitive in Canada, which prides itself as a nation of immigrants that offers refuge from political, ethnic and religious persecution.

Prime Minister Jean Chretien's government has been under pressure to tighten immigration and refugee policies because of evidence that a few of the more than 200,000 newcomers a year import terrorism.

The Canadian Security Intelligence Service has warned repeatedly that terrorist groups operate in the country. Last year, Canada was embarrassed by news that Ahmed Ressam, arrested in Washington state and convicted this year of plotting a foiled terrorist bombing of Los Angeles International Airport, lived in Montreal for years despite a deportation order.

Although no Canadian links have been found to last week's hijackers, some members of the U.S. Congress call for tightening the border.

"There is no question that Canada is a soft spot. Canada has been used as a staging ground," Rep. Rick Larsen, a Democrat from Washington, said Tuesday. "Unless changes are made in the future in our law enforcement relationship with Canada, it will continue to be."

The Globe and Mail newspaper reported Wednesday that Canada has ordered its border inspectors to be especially vigilant for people from certain Muslim countries and those with backgrounds in aviation, nuclear engineering and other scientific fields.

An Immigration Canada spokesperson said border inspectors have received new instructions since last week's attacks, but he refused to discuss details.

Cellucci and others argue an open border is crucial to maintaining the world's largest trade partnership, with goods, services and people going in both directions each day creating a mutual economic dependency.

"To shut the border or impose major restrictions would end up amputating the right arm of the U.S. economy," said Chris Sands of the Center for Strategic and International Studies in Washington.

Sands said minimum probable changes in border security would include better screening, with passports required, and special bridges for trucks and other commercial traffic. Since the attacks last week, increased security has caused lines of trucks stretching for miles at some border points.

"Even if we came up with a force field in the United States, we'd still run into the problem of Canadian exposure," he said. "This is a psychologically linked population, and the notion that one side can be protected without the other should be changing, if it's not already."

Joining a perimeter-security plan with its dominant neighbor would require Canada's immigration and refugee policies to conform to tougher U.S. guidelines, increasing an already dreaded perception that Canada is losing its sovereignty.

Chretien insists that Canada can maintain valued principles while joining the U.S. battle against terrorism.

"Perhaps there will be a need of changing some laws," he said Tuesday. "We want to improve them and do what is right, but in the spirit of maintaining the type of society we have in Canada where we respect the freedom ... and movement of people."

To Cellucci, the new challenge of terrorism requires new thinking.

"We must also work our way through now old arguments of security versus sovereignty," he said, "for what is sovereignty without security?"