OTTAWA -- Conservatives and New Democrats are joining forces in a bid to pressure the Trudeau government to reveal its priorities for the imminent renegotiation of the North American Free Trade Agreement.

Four MPs from both parties sent a joint letter Tuesday to the clerk of the Commons international trade committee, requesting that an emergency meeting be held to grill three cabinet ministers on Canada's negotiating objectives and "expected positive outcomes" of the renegotiation, which is to begin in mid-August.

The committee, on which Liberals hold the majority, is now scheduled to meet Friday to decide whether to accede to the request.

If members agree, the ministers could be called to testify immediately.

Insiders said that Foreign Affairs Minister Chrystia Freeland, the lead minister on NAFTA negotiations, is willing to appear before negotiations start next month but is unlikely to be available as early as Friday.

The request comes one day after the United States disclosed its negotiating objectives, as is required under American law.

There is no equivalent requirement in Canada and the Trudeau government has so far refused to go into any detail about its priorities, maintaining that it's not helpful to negotiate in public.

But the opposition parties want to force the government's hand by inviting three key ministers to testify at an emergency committee meeting -- Freeland, International Trade Minister Francois-Philippe Champagne and Finance Minister Bill Morneau.

As well, they're asking that Canada's chief NAFTA negotiator, Steve Verheul, be present.

Conservative MP Randy Hoback, one of the four committee members requesting the emergency meeting, said the meeting is necessary to make sure the government has a game plan for negotiations, that they've identified the issues Canadians want to see on the bargaining table and those they don't want on the table.

"We're not asking them to negotiate in public," he said in an interview.

"But we definitely want to see that they've got a process in place. We know they've been out talking to Canadians ... that they've got an idea what they would like and not like in these negotiations."

Hoback said the government could at least identify "what are the do-not-touch items."

Prime Minister Justin Trudeau took a step in that direction Tuesday, indicating that he won't bargain away Canada's supply management system for dairy and poultry products, which U.S. President Donald Trump has repeatedly targeted as unfair subsidization.

"We have always defended supply management. It's a system that works," Trudeau said during a visit to Quebec.

"We have signed significant trade deals with Europe, with North America, elsewhere, protecting our supply management system and we are going to continue to do that."

Hoback said he understands the government's reluctance to give away its negotiating strategy. But when there's too much secrecy, he argued that "assumptions can be made that aren't necessarily true" which can shake investor confidence and create instability for business.

Moreover, he said it allows the U.S. to drive the agenda.

Among other things on the U.S. wish list revealed Monday, the Trump administration wants better access for U.S. agricultural exports, freer trade in telecommunications and online purchases and an overhaul of the dispute resolution mechanism.

Morneau said there were no surprises in the list.

"As you know we've been in pretty regular contact with our American counterparts," the finance minister said in an interview.

"We, as a result of that, had a good sense of what they're trying to achieve. There wasn't anything in there that was a big surprise from my perspective."

Morneau acknowledged that there will be "significant negotiations ahead and we shouldn't expect that we're going to agree on everything."

But he expressed optimism that the relationships the Trudeau government has built with the Trump administration, Congress, state legislators and American business and labour leaders will help Canada negotiate a new deal that is an improvement for all three countries.