Prime Minister Justin Trudeau called U.S. President Donald Trump Monday evening to talk about the president's vow to impose steep tariffs on imports of steel and aluminum.

Trudeau spoke with Trump about the closing of the latest round of NAFTA negotiations in Mexico City, flagging the progress that had been made in this seventh round of talks while expressing concerns about the threat of tariffs.

"The prime minister also registered his serious concern about the U.S. administration's proposed tariffs on steel and aluminum," said a read-out of the call from the Prime Minister's Office.

A government source with direct knowledge of the trade file told CBC News that, during the call, Trudeau made the case for Canadian steelworkers and argued that tariffs would frustrate ongoing NAFTA negotiations.

The source said there now appears to be a sense of urgency on the U.S. side about the NAFTA talks — that American negotiators want to wrap up the negotiations soon, to avoid any possible disruptions from Mexico's July presidential elections.

Last week, the Trump administration announced it would use section 232 of the Trade Expansion Act of 1962 to impose tariffs on steel and aluminum imports on the argument that they threaten U.S. national security.

Trump said he would impose the tariffs — 25 per cent tariff on steel and 10 per cent on aluminum, — to boost U.S. manufacturers. The president also suggested that Canada might get an exemption if it agrees to a "new and fair" NAFTA deal with the U.S.

A sudden sense of urgency

Earlier Monday in Mexico City, where Canada, the U.S. and Mexico were wrapping up the seventh round of NAFTA talks, U.S. Trade Trade Representative Robert Lighthizer suggested that time is running out for the three countries to strike a deal.

Lighthizer pointed to the Mexican elections, and the provincial elections in Ontario and Quebec later this year, as examples of events that could derail NAFTA talks.

"All of this complicates our work. I fear that the longer we proceed, the more political headwinds we will feel," he said.

Foreign Affairs Minister Chrystia Freeland said she has been pressing for Canada to be excluded from any steel or aluminum tariffs.

Foreign Affairs Minister Chrystia Freeland, left, Mexico's Secretary of Economy Ildefonso Guajardo Villarreal, center, and U.S. Trade Representative Robert Lighthizer pose for a photo at a press conference after wrapping up the seventh round of NAFTA renegotiations in Mexico City, Monday. (Marco Ugarte/Associated Press)

"We did have a conversation with Ambassador Lighthizer about section 232," she said. "It was a useful conversation and I'd like to thank Ambassador Lighthizer about how informed he was on the issue, including Canadian specifics."

Canada is ready to respond to tariffs, Freeland said. She would not say how.

"I do not judge that this is the moment for us to be talking about the specifics of our responsive measures," she said. "If necessary we would consider any section 232 action directed at Canada to be totally unacceptable and in that event we would take responsive measures to defend our workers and our industry."