Canada's top trade negotiator said on Friday she and her US counterpart were making "very good progress" in talks to save the North American Free Trade Agreement amid increasing Canadian optimism that a deal could be reached, even if a conclusion did not appear imminent.

Canadian Foreign Minister Chrystia Freeland repeated her upbeat assessment of the negotiations, again terming them constructive, as she spoke to reporters after a meeting in Washington with US Trade Representative Robert Lighthizer.

"We are certainly making very good progress at understanding each other, understanding what each side needs," Freeland said.

But the talks on Friday were not headed toward an immediate conclusion. Freeland was due to fly out of Washington on Friday evening, a senior Canadian official said, adding that further minister-level discussions would not likely be held until sometime next week.

Lighthizer is scheduled to be in Brussels on Monday for trade talks with the European Union's trade commissioner, Cecilia Malmstrom, and is due back in Washington on Tuesday.

The main sticking point in the US-Canada Nafta talks appeared to be Canada's dairy quota regime, according to the White House's top economic adviser, Larry Kudlow, who spoke before the start of the latest talks.

Kudlow spelled out "M-I-L-K" in an interview with Fox Business Network

for emphasis.

President Donald Trump has struck a trade deal with Mexico and threatened to push ahead without Canada, a move that would kill Nafta, which covers $1.2 trillion in trade between the three countries, and further spook financial markets.

Other sticking points include Ottawa's desire to keep the 1994 pact's Chapter 19 dispute resolution mechanism, and Canadian media laws that favor domestically produced content.

"I'm working on Canada now," Trump told reporters aboard Air Force One "You know, people can say, 'Oh I'm too tough on Canada.' Look, Canada's been ripping us off for a long time. And now they've got to treat us fairly."

A Canadian source who declined to be named given the sensitivity of the situation, said Canadian negotiators thought it was quite possible the talks would continue until the end of this month.

A US official told Reuters on Thursday that Canada needed to move further on dairy. In its recent trade deal with the European Union, Canada made concessions on dairy imports and it offered some increased access to that market as part of the Trans-Pacific Partnership, which the United States abandoned.

"We're down to three issues: Chapter 19, the cultural issues and dairy. We've created leverage and driven Canada to the table," the US official said. "Part of our problem is that Canada has been backsliding on its commitments (on dairy)."

Trump has targeted what he sees as

"unfair" trade as part of his "America First" agenda to boost US manufacturing and jobs, imposing tariffs on tradingpartners, including Canada, China, the EU and Mexico. That has prompted retaliation.

Tens of billions of dollars in Chinese imports have been slapped with US tariffs and a new round of duties is due to be triggered soon.

Both Canada and Mexico want Trump to agree to permanently exempt them from US tariffs on steel and aluminum imports. Washington has used those tariffs as leverage in the Nafta talks.

For its part, Canada has used the provisions of Nafta's dispute resolution mechanism to defend its lumber exports to the United States. Washington charges that Canadian lumber unfairly undercuts prices on US lumber.

Negotiators and their staff have held several late-night sessions in a bid to overcome disagreements this week.

"We are really in a continuous negotiation phase," Freeland said.

The Republican chairman of the US House of Representatives Ways and Means Committee, Kevin Brady, a powerful voice in Congress on trade, told reporters differences remained between the two sides over Canada's dairy quota regime, a trade dispute resolution settlement procedure and "other longstanding issues."

"My sense is that everyone is at the table with the intention of working these last, always difficult issues out," Brady told reporters after speaking with Lighthizer on Thursday.