OTTAWA -- Smugglers were caught trying to slip dramatically more people into Canada in 2011 over the previous year at largely unguarded points along the border with the United States, says a newly declassified report.

Authorities apprehended 487 people as smugglers attempted to sneak them into Canada at remote locales, up from 308 in 2010, says the binational report on border security.

At the same time, the number of people nabbed while being spirited into the United States from Canada fell slightly during the same period to 360 from 376.

The figures on smuggling between official ports of entry appear in the 2012 Integrated Border Enforcement Team threat assessment report, obtained by The Canadian Press under the Access to Information Act.

The report indicates that stronger enforcement may partly account for the increase in arrests, but adds there is concern that "a significant portion" is the result of "a surge in human smuggling activity."

An advocacy group for refugees attributes the 58-per-cent rise in Canada-bound human smuggling attempts to an agreement between the countries that has prompted desperate refugees to turn to criminal groups willing to help them across the border.

Under the Safe Third Country agreement, which took effect in December 2004, Canada and the U.S. recognize each other as safe places for refugee claimants to seek protection.

It means Canada can turn back potential refugees at the Canada-U.S. border on the basis they must pursue their claims in the U.S., the country where they first arrived.

Critics have fought the agreement, arguing the U.S. is not always a safe country for people fleeing persecution. They point out that more restrictive American rules and interpretation of who qualifies as a refugee have meant that some claimants rejected by the U.S. were later accepted by Canada.

"The U.S. laws mean that some people who would get protection in Canada do not get protection in the U.S.," said Janet Dench, executive director of the Canadian Council for Refugees.

"It serves the interests of criminal groups who then have an opportunity to exploit people who need to get here."

The 23 binational border enforcement teams operating in 15 regions between ports of entry include members of the RCMP, the Canada Border Services Agency, the U.S. Customs and Border Protection Service, the U.S. Coast Guard and other American agencies.

They try to foil smugglers of people, drugs, currency, firearms and other commodities.

"The sheer size and diverse terrain of the border pose a daunting challenge to enforcement strategies in the quest to counter cross-border criminal activity," says the report -- portions of which were deemed too sensitive to release.

It says 115 criminal groups and 106 "criminal entrepreneurs" used a variety of means -- from all-terrain vehicles and cross-country skis to kayaks and helicopters -- in their efforts to get cargo across the border in 2011.

"If a crime group or criminal entrepreneur is involved in the smuggling of migrants the typical mode of transport used is a combination of vehicle and on-foot land transport," the report says.

The smugglers will provide migrants with a car ride to a point near the border and the people are then told to walk -- often through the woods -- to the other side, where a waiting taxi or private vehicle picks them up and heads further inland, the report adds.

It also notes a sizable increase in the number of people caught trying to slip across the border from the U.S. on their own -- to 762 in 2011 from 437 the year before.

The oil boom on both sides of the 49th parallel has sparked an increase in the Prairie region of this kind of surreptitious movement -- likely by people seeking jobs -- as well as drug trafficking, the report says.

The figures come as the two countries aim to build on cross-border co-operation by creating new integrated teams in areas such as intelligence and criminal investigations -- part of a sweeping Canada-U.S. perimeter security pact.