The insurgency in Afghanistan will never be defeated only by maintaining an international troop presence in the country, Prime Minister Stephen Harper said in a U.S. television interview Sunday.

"We're not going to ever defeat the insurgency. My reading of Afghanistan in history is that it's probably had an insurgency forever of some kind," Harper told Fareed Zakaria of CNN.

Harper wouldn't say absolutely whether Canada would agree to an extension of its combat mission, set to end in late 2011, but he did say emphatically that Afghanistan cannot be governed or controlled by any foreign power indefinitely.

"Ultimately the source of authority in Afghanistan has to be perceived as being indigenous," he said. "If it's perceived as being foreign — and I still think we're welcome there — but if it's perceived as being foreign, it will always have a significant degree of opposition."

Harper said he welcomes plans to boost the number of U.S. troops in Afghanistan, but was cautious about entertaining any request for Canada to alter its commitments. On Feb. 17, U.S. President Barack Obama said he'll send an additional 17,000 American soldiers to Afghanistan this spring and summer, a 50 per cent increase to the 36,000 soldiers who are there already.

Harper said if Obama were to ask Canada for a larger contingent or a continuation of the existing contingent of about 2,500 Canadian soldiers, he would ask the president what his plans are for leaving Afghanistan and allowing Afghans full control over security.

"If we think that we are going to govern Afghanistan for Afghans, or over the long-term be responsible for day-to-day security in Afghanistan and see that country improve, we are mistaken," Harper said.

Tories have come around: NDP

The NDP and its leader, Jack Layton, were attacked by the government for saying that fighting the insurgency is unwinnable.

"They called him Taliban Jack," NDP MP Dawn Black told CBC News on Sunday. "They called me Burka Black, and now after all these years — three years, the many sacrifices, the enormous cost — Mr. Harper is saying exactly what many people have been saying over the years."

The Liberals supported the Conservatives' decision to extend the Afghanistan mission, but they too complain that Harper has had the emphasis wrong.

"It's clear that Mr. Harper's coming to the right conclusion, but he's not following it up with the right kinds of actions," Liberal MP Bob Rae said Sunday. "And he's not taking the steps that would be logical for Canada to take, which is to upgrade its diplomatic effort and its political effort and its development effort."

The idea the insurgency will never be defeated is not new and the United States, which is increasing its military efforts there, is careful to define limited objectives, the CBC's Keith Boag said.

"Afghanistan is still winnable in the sense of our ability to ensure that it is not a launching pad for attacks against North America," Obama said in an exclusive interview with the CBC's Peter Mansbridge last month. "I think it's still possible for us to stamp out al-Qaeda."