MEXICO CITY — A top Mexican trade negotiator flew to Washington on Sunday for urgent talks as a hitch emerged in the U.S.-Mexico-Canada Agreement, just days after it was signed.

Jesús Seade, undersecretary for North America in the Foreign Ministry, accused the United States of blindsiding Mexico by deciding to send up to five U.S. attaches to monitor labor conditions as part of the treaty.

That decision was included in implementing legislation sent to the U.S. Congress on Friday. The new treaty is intended to replace the North American Free Trade Agreement.

Mexico’s labor practices were a major sticking point in the final rounds of negotiations on the accord. U.S. unions — and their allies in the Democratic Party — pushed for tough enforcement of a new Mexican law that guarantees workers the right to elect their leaders and approve contracts. In the past, Mexican unions were often under the thumb of businesses and politicians, who kept a lid on workers’ wages.

During the talks, Mexico rejected a U.S. proposal for foreign labor inspectors, saying it would violate the country’s sovereignty. Instead negotiators agreed to establish three-member panels — made up of Mexican, American and other experts — to resolve disputes.

Seade said he had sent a letter to Robert E. Lighthizer, the U.S. trade representative, expressing “Mexico’s surprise and concern” about the language sent to Congress.

The decision to send labor attaches was “never mentioned to Mexico — never,” Seade told journalists on Saturday. “And, of course, we don’t agree.”

On Sunday, he was even more blunt. “We gained a lot in the trilateral talks, and because of this, the U.S. needs ‘extras’ that are NOT PART OF THE TREATY in order to sell it to its domestic audience,” Seade tweeted.

The U.S. trade representative’s office did not immediately respond to a request for comment.

But Lighthizer told CBS News’s “Face the Nation” on Sunday that the treaty “was more enforceable and it’s better for American workers and American manufacturers and agriculture workers than it was before.”

Mexico’s Senate voted overwhelmingly Thursday to approve the treaty, just two days after it was signed by the three nations’ negotiators in the Mexican capital. But the labor issue has since blossomed into a political controversy here. Critics have charged that Seade was careless or naive.

“It was a serious error for Seade to have gone alone to the final negotiations on USMCA,” José Antonio Crespo, a political scientist at the Center for Research and Teaching in Economics, wrote on Twitter. “If he had been advised by Mexican personnel, he wouldn’t have been tricked, or be pretending that he’d been tricked.”

Seade said Mexico would never accept foreign labor inspectors “for a simple reason: Mexican law doesn’t allow them.” The Foreign Ministry noted in a communique that Mexico could reject any such diplomats the United States sought to post in the country.

Mexico’s economy is heavily dependent on foreign investment and trade with its northern neighbor, the market for 80 percent of its exports. For that reason, the leftist government of President Andrés Manuel López Obrador has been strongly supportive of negotiations to create a successor to the 25-year-old NAFTA.

“Mission accomplished,” Foreign Minister Marcelo Ebrard said last week.

Alone among the three nations, Mexico this year ratified an initial version of USMCA negotiated last year. López Obrador’s Morena party controls Mexico’s senate, which is charged with ratifying treaties.

The United States and Canada sought revisions, leading to the deal approved last week. Those countries have yet to ratify it. President Trump will need support from House Democrats, and Canadian Prime Minister Justin Trudeau, who leads a minority government, will need some backing from other parties.