With the deadline looming for Canada to decide whether to extend its 30-day military advisory mission in Iraq, the opposition is accusing Prime Minister Stephen Harper of holding back key information on military talks with the United States.

A spokesperson for the U.S. Secretary of Defense said Thursday that Canada sent the U.S. a letter asking for more details on how it could contribute to coalition efforts in the fight against Islamic State militants in Iraq.

The U.S. then replied with a letter describing ways Canada could provide assistance, according to the DoD.

Prime Minister Stephen Harper told a business audience in New York City Wednesday that the U.S. had asked “for some additional contribution” and that his government is “weighing our response.”

CTV News has learned that Canada did receive an initial letter from the U.S., but it was a form letter sent to all allies in mid-September, not a special request to the Harper government.

In a statement issued late Thursday, Liberal Leader Justin Trudeau accused Harper of “playing coy political strategy games and fudging the truth.”

“It now appears as though the Prime Minister omitted a key detail about the American request for further Canadian participation in Iraq,” Trudeau said. “It was not the U.S. asking Canada for help. It was the other way around.”

But Harper’s director of communications, Jason MacDonald, told The Canadian Press that Canada has always said it would listen to requests for help in the fight against ISIS.

"It was in response to those conversations that we received a letter," MacDonald said.

In his statement Thursday, Trudeau also called on Harper to present his plan for Canada’s involvement in Iraq.

“Any change to Canada’s non-combat mission in Iraq must only be made with rigorous and effective Parliamentary oversight,” Trudeau said. “We expect the Prime Minister to answer all questions pertaining to this deployment fully and openly in the House of Commons in front of Canadians.”

Foreign Affairs Minister John Baird said Thursday that the issue would be put to a vote in the House of Commons.

"If there's a combat mission, I think the prime minister has always been very clear, that would go before Parliament for a vote, that has not changed," Baird told reporters at the United Nations.

MPs demand answers

Earlier Thursday, NDP Leader Thomas Mulcair and other opposition MPs demanded to be informed of the contents of the letter requesting Canada’s help in Iraq.

Mulcair also noted that Harper had previously said that Canadian military missions would require a vote in the House.

“There’s been no debate organized by them, no real information given to Canadians or to Parliamentarians, and there’s no vote,” Mulcair told CTV’s Power Play Thursday evening.

“We’re concerned about that.”

Mulcair added: “What’s more important that you can decide in the life of your Parliament or your country than whether or not you send women and men to fight in a war where they could die? So this is a very serious matter and it’s something that requires the proper attention based on full information.”

Earlier in question period, Mulcair had called on the federal government to go public with the letter that U.S. officials sent with the troop request, so Canadians and Parliamentarians can be clear of its nature and timing.

“How many additional soldiers were requested by the United States and how many additional soldiers does the prime minister intend to send?” Mulcair asked.

Defence Minister Rob Nicholson said the request for additional support “will be part of the review by the government at the end of the 30-day period, and we’ll make a decision.”

Liberal MP Marc Garneau asked whether the government will consult Parliament if it considers expanding Canada’s role in Iraq. Nicholson did not respond to the request directly, saying only that the government has been forthcoming with mission details in both briefings to the opposition and in committee.

Nicholson responded to a similar question from MP Joyce Murray by adding that the NDP and the Liberals can use their opposition days in the House to launch a debate on the Iraq mission.

Mulcair took another turn and asked directly for a vote in the House, a request Nicholson did not directly address.

After several days of questions in the House about when Canada’s mission in Iraq officially began, Parliamentary Secretary to the Defence Minister, James Bezan, confirmed Wednesday that the “clock started running” on Sept. 5.

That leaves just over a week to go before the government must decide whether to extend the mission. Canada currently has 69 special ops soldiers in Iraq, training and advising Iraqi soldiers in their fight against Islamic State militants.

A senior government source told CTV News Wednesday that Canada is considering doing more to support the fight, but the source said Canada will not be committing troops to a combat role.

The prime minister avoided commenting on ISIS during a speech to the United Nations General Assembly on Thursday, instead opting to focus on child and maternal health.

With a report from CTV's Deputy Ottawa Bureau Chief Laurie Graham and files from The Canadian Press