"Our relationship with Mexico is getting closer by the hour", Trump wrote.
The federal government insists it hasn't been frozen out of the talks, and Foreign Affairs Minister Chrystia Freeland said she's been in regular contact with both her counterparts, U.S. Trade Representative Robert Lighthizer and Mexican Economy Minister Ildefonso Guajardo.
Seade said he and Lighthizer last week began discussing a new review approach, one that would have longer periods between reviews, providing more certainty for business investments. They include the USA proposal that NAFTA include a five-year sunset clause, which has been flatly rejected by the Trudeau Liberals.
Earlier in August, Trump threatened Canada with auto tariffs if the US and Canada can't forge a deal. Guajardo has consistently said that he expects the issue to be one of the final ones to be ironed out, because it requires Canada's participation.
The negotiations are aimed at ironing out bilateral issues and then finalizing the new agreement with Canada, the third NAFTA partner.
Shortly after Trump's tweet, the incoming chief trade negotiator for Mexican President-elect Andres Manuel Lopez Obrador, Jesus Seade, said the U.S. has softened its position on the "sunset clause" in NAFTA.
A report by Politico, based on information from unidentified sources, said the White House is expected to announce a "handshake" deal between the USA and Mexico on Thursday.
Asked about a potential deal on Monday, he said "That's why we're here; to make our best effort".
Speaking to reporters on his way into Saturday's talks in Washington, Guajardo praised the U.S. president's "optimism".
Seade also predicted that the nations will agree on a method to update Nafta without the threat of a so-called "sunset clause", an automatic expiration after five years - as the U.S. has sought since October.
However, representatives of US agricultural, manufacturing and other sectors have cautioned the White House that walking away could be a blow for the USA economy.
"We're trying to really get all the solutions that are required, and we're well advanced, but we're not there yet", Guajardo said.
Since Mexico's July 1 presidential vote, the bilateral talks have been complicated by the divisions between the incoming and outgoing Mexican administrations over the energy policy.
Mexico's Economy Minister Ildefonso Guajardo told reporters on Sunday a deal between the us and Mexico was nearing completion. The US and Mexico are pushing for an agreement this month that would give the countries time to sign the pact before Mexico's President-elect Andres Manuel Lopez Obrador takes office in December.