The U.S. border with Canada, even though it sees far fewer detentions and arrests every year, is a "more significant threat" to American security than the Mexican border, a senior Homeland Security official said Tuesday.

U.S.-Canada border security, like at this crossing in Blaine, Wash., has been under scrutiny in recent months following a stunning finding in a U.S. government report. Elaine Thompson/Associated Press

"In terms of the terrorist threat, it's commonly accepted that [it's] the more significant threat, because of... people who can enter Canada then and come across our bridges into the United States," Alan Bersin, head of the U.S. Customs and Border Protection agency, told a U.S. Senate subcommittee hearing in Washington.

Bersin was responding to questioning from Senator John Cornyn, a Texas Republican who demanded to know why the southern U.S. border, with about 445,000 arrests and detentions annually, isn't considered far more sensitive than the northern boundary, which saw 6,000 arrests and detentions last year.

Bersin said the U.S. has to "maintain a very high security profile" on the Canadian frontier, in part because Canada and the U.S. don't share their no-fly lists and so can't keep out people deemed security threats who might travel by airplane into one country, then cross the border by land.

American politicians and officials have often expressed alarm about their northern boundary, frequently stemming from the false belief that some of the 19 men who hijacked four airplanes on Sept. 11, 2001, entered the United States from Canada. In February, several senators decried the findings of a government report that determined that less than one per cent of the U.S.-Canada border had reached an "acceptable level of security."

Senators at Tuesday's hearing also asked what efforts are afoot to stem drug smuggling between Canada and the U.S.

The Customs and Border Protection chief replied that the latest step in cross-border policing will see Canada send 22 radar feeds to his agency's centre in California that tracks non-commercial aircraft, with the goal of cracking down on narcotics smuggling by small planes flying low over the border to escape detection.

Train travel concerns

In other testimony, Bersin said U.S. officials are working with the Canada Border Services Agency to permit pre-trip immigration clearance for train travellers from Montreal to New York.

Senators Charles Schumer of New York and Patrick Leahy of Vermont, both Democrats whose states see significant tourism from Canada, had expressed frustration at delays on the Montreal to New York City line, with customs and immigration checks at the border stalling trains by up to two hours on their way south.

"Can we count on you to speed up the trains between Canada and New York?" Schumer asked.

Bersin said one complication on rail company Amtrak's route is that it stops south of Montreal before hitting the border, making it difficult to properly pre-clear all passengers. His agency already does pre-screening at Vancouver's train station for Seattle-bound trips, but those trains don't stop elsewhere in B.C.'s Lower Mainland before crossing into Washington state.