Even as President Donald Trump congratulated Mexico’s president-elect in a letter, he warned that a successful renegotiation of the North American Free Trade Agreement must be done quickly.

If progress is not made, Trump said, “I must go a much different route. It would not be my preference, but would be far more profitable for the United States and its taxpayers.”

The letter to Andres Manuel Lopez Obrador, who emerged victorious in Mexico’s presidential election on July 1, also mentioned immigration.

“Like you, I believe that meeting the challenge of illegal immigration involves more than just strong border security. We are prepared to further address the economic development and security issues that drive migration from Central America, but we must also increase cooperation to protect the rule of law and the sovereignty of both our countries.”

Trump concluded that “a strong relationship will lead to a much stronger and more prosperous Mexico, which frankly would make me very happy.”

The letter, dated July 20. was read aloud at a press conference on Tuesday by Marcelo Ebrard, Lopez Obrador’s choice to become his government’s foreign minister. It was sent in response to a letter that Lopez Obrador had sent to Trump.

Ebrard remarked that it’s “very valuable” that the American president expressed a willingness to work together on economic development.

The letter emerged as high-level Nafta talks pick up again this week in Mexico City following two months of limited negotiations that were marred by the diplomatic fallout from tariff battles. The negotiations had also been put off in the lead-up to Mexico’s election.

Ebrard added that the incoming government would participate in the talks.

The White House did not respond to a request for comment on the letter on Tuesday night.