Secretary of State Hillary Clinton, seen last month in Haiti, says the U.S. wants its northern border to be 'as secure as possible.' (Ramon Espinosa/Associated Press)

The Canada-U.S. border has been a problematic one, Secretary of State Hillary Clinton said Tuesday, which is why both countries are working together to "harden" the international boundary.

"Americans are worried about every port and point of entry; I don't think we have any lesser concern about any other route into our country than any other one," Clinton told a news conference in Washington, D.C.

Clinton was responding to a question about why so many Americans still believe the Sept. 11, 2001, hijackers entered the U.S. through Canada — an oft-repeated myth that she herself has in the past stated as fact — but didn't answer it directly.

"Obviously, we're proud of the long, peaceful border that we share with Canada, but I think it is fair to say that since 9/11, we have been working with our friends in Canada to try to harden that border, to try to provide both more personnel and technology," she said.

"Unfortunately, given the security environment that we have to deal with today, we have been focused on making sure that our northern border was as secure as possible."

Clinton was referring to the Western Hemisphere Travel Initiative, a measure that will require all travellers to the U.S. to carry a passport or similarly secure document as of June 1.

The challenge in developing a Canada-U.S. border policy, Clinton said, is achieving security "without undermining either our relationship or the trade in goods and services, the tourism, the natural flow of people who both work and go to school and recreate on both sides of the border."

Nonetheless, many observers in Canada have fretted that the U.S. is formulating border policy based on fallacies about their mutual boundary.

Homeland Security Secretary Janet Napolitano suggested last month, and then retracted, that the Sept. 11 hijackers came from Canada.

She also said Canada's immigration regulations are far more lax than those in the U.S., and said Canadian authorities allow people into the country who would never be granted entry south of the border.