Talks not living up to expectations - US negotiator

"In spite of (our) hard work, we have not made the progress that many had hoped in this round. We have closed out only three additional chapters," said Robert Lighthizer, the chief US negotiator, as the seventh round of NAFTA talks wrapped up in Mexico City.

"To complete NAFTA 2.0, we will need agreement on roughly 30 chapters. So far, after seven months, we have completed just six," he told reporters.

The warning came after Trump vowed Canada and Mexico would also be affected by his plans to impose steep tariffs on steel and aluminum unless he gets a "fair" deal to overhaul NAFTA.

"Tariffs on Steel and Aluminum will only come off if new & fair NAFTA agreement is signed," he said in one of a series of morning tweets.

He later doubled down on the threat in a White House press conference.

"No, we're not backing down," he said. "There will be tariffs on steel for Canada and for Mexico."

Canada's Foreign Minister Chrystia Freeland vowed in her own remarks at the close of the NAFTA talks that her country would fight fire with fire.

"Should restrictions be imposed on Canadian steel and aluminum products, Canada will take appropriate, responsive measures to defend our trade interests and our workers," she said.

Lighthizer said it is urgent to update the 1994 deal, warning that a July presidential election in Mexico plus November mid-term elections in the United States are complicating the talks.

"As President Trump has said, we hope for a successful completion of these talks and we would prefer a three-way, tripart agreement. If that proves impossible, we are prepared to move on a bilateral basis," he said.

Trump triggered the renegotiation of NAFTA, which he has called the worst deal the US ever signed, shortly after taking office.

Mexican Economy Minister Ildefonso Guajardo said the three countries would hold a series of "inter-session" talks on the most "complex issues" before the next formal round of negotiations, expected to be held next month in Washington.

His country sends some 80 percent of its exports to the United States.