OTTAWA -- Foreign Affairs Minister Chrystia Freeland says serious challenges remain in dealing with U.S. proposals in the ongoing NAFTA negotiations.

"Serious challenges do remain, particularly with regard to the United States' unconventional proposals," Freeland told the House of Commons foreign affairs committee Thursday.

She said Canada continues to pursue a "fact-based approach" to the negotiations and has put forward "creative ideas" to resolve outstanding issues on autos and the investment dispute resolution mechanism.

Her testimony came on the heels of reports suggesting growing U.S. frustration with Canada.

A congressman who was among lawmakers briefed by Robert Lighthizer this week said the trade czar was frustrated with Canada and was considering concluding a quick agreement with Mexico before dealing with Canada.

The U.S. trade representative later issued a statement reiterating his commitment to the three-country continental accord.

Lighthizer expressed dissatisfaction at the end of recent NAFTA negotiations in Montreal over some of Canada's proposals, calling one of them a "poison pill."

He also accused Canada of launching a "massive attack" on the U.S. trade system by filing a detailed complaint about punitive American trade practices to the World Trade Organization in December.

Earlier this week, Ontario threatened retaliation over Buy America provisions, saying it would introduce legislation that would allow retaliation against U.S. states that adopt such policies.