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Reports that the United States may ask Canada to keep between 500 and 600 troops in Afghanistan after the military deployment ends next year sparked a fiery question period Thursday, as opposition parties pressed the government for answers.

It's not known when U.S. President Barack Obama will speak to Prime Minister Stephen Harper about the issue. But government sources said Harper will not agree to the request, Ottawa Bureau Chief Robert Fife reported Thursday night.

Taking the floor of the House of Commons on Thursday, Liberal Leader Michael Ignatieff criticized the way the potential request entered the public sphere of discussion.

"The government will have to explain why this obvious trial balloon entered the press," he said. "The government didn't bring this before the Canadian people -- this is no way to conduct foreign policy. Will the government commit to put any deployment in Afghanistan past 2011 to a vote in Parliament?"

The troops would act as military trainers and would likely be stationed in and around Kabul. The request is expected to be made through NATO by the end of the year, the Globe and Mail reported Thursday.

Foreign Affairs Minister Lawrence Cannon told the House that such a request has not been made.

"I want to assure the leader of the Opposition that we have not received a request of this nature," Cannon said.

International security analyst Alan Bell said Thursday that even in a training role, Canadian troops in Afghanistan would likely still be in danger.

"If you are training the Afghan army, and you are mentoring them, that means you have to go out on operations into the field with them," Bell told Â鶹ӰÊÓ Channel. "It's going to be impossible for the Canadian army to offer a mentoring role and not be put in a position of danger."

Responding to questions from the opposition parties during question period Thursday, Cannon maintained that the government is committed to ending Canada's military mission in Afghanistan in 2011.

"Our military mission will end in 2011 as we've indicated in the speech from the throne," Cannon said in French. "Canada will continue diplomatic and development relationships with Afghanistan through its embassy in Kabul as we did in other countries."

CTV's Tom Clark said Thursday that if the request for extended troops made its way to Parliament, it would likely put more pressure on the opposition than on the Conservative government.

"If you float it in Parliament, this puts pressure on the Liberals," he told Â鶹ӰÊÓ Channel Thursday afternoon.

It was a Liberal government that first approved sending Canadian troops to Afghanistan, but in subsequent votes over the mission in Afghanistan, Liberal MPs have expressed differing views on the subject.