OTTAWA — Prime Minister Stephen Harper says there's no mystery why Canadian troops are at greater risk in Afghanistan than other NATO soldiers _ they're in the most dangerous area of the country.

"We are taking casualties because we are moving (the Taliban) from their very last bastions of strength and support,'' Harper told CTV NewsNet on Monday.

"It's a tough mission, but the men and women of the Canadian Forces sign on for tough missions if they know they can do good in the world _ that's what they're doing and ... they have the absolutely unwavering support of their government.''

Harper was responding to a study by the Canadian Centre for Policy Alternatives which concludes that Canadian soldiers in Afghanistan are six times more likely to be killed in a hostile attack than are American or British troops.

The report says that if the recent rate of casualties continued until the end of the current mission in January 2009, there would be 108 further deaths.

Four more Canadian soldiers were killed by a suicide bomber in southern Afghanistan on Monday, raising the total death toll to 36 since 2002. A Canadian diplomat has also died.

"As we examined the troubling data, the question arose as to whether the Liberals misjudged the danger, and if the Conservatives ignored it,'' said Steven Staples, co-author of the report.

Harper reiterated that while the Afghan mission technically ends in 2009, it may be extended again.

Canadian troops will remain in the country until they have achieved "success'' by putting in place the proper conditions for stable democracy, he said.