President Trump renewed a threat Monday to make tighter control of the southern border a condition of broad trade deal that his administration is renegotiating with Canada and Mexico, saying, “Our Country cannot accept what is happening.”

“Mexico, whose laws on immigration are very tough, must stop people from going through Mexico and into the U.S.,” Trump wrote on Twitter. “We may make this a condition of the new NAFTA Agreement.”

The president voiced similar concerns earlier this month, claiming that Mexico is doing little to stop the flow of illegal immigrants and drugs into the United States and suggesting that he could use the North American Free Trade Agreement as leverage on those issues.

Trump’s latest threat comes as trade negotiators are already grappling with thorny issues amid a rush to conclude at least the outlines of a deal in the next few weeks — a process that could be upended if Trump is serious about adding a border-crossing condition.

The White House and the Office of the U.S. Trade Representative did not immediately respond to a request for comment on whether the president had directed his negotiators to add language addressing that to the NAFTA talks.

For the Republican-controlled Congress to vote on the deal, the process must begin soon, perhaps by reaching an agreement in principle that would cover the main provisions of a new deal, U.S. officials have said. Intense talks in recent weeks have made progress on the core issues, including rules for determining which automobiles are entitled to duty-free treatment under the accord.

But negotiators have officially completed only six of 30 treaty chapters, and remain deadlocked on a handful of contentious U.S. proposals, including on government procurement, intellectual property, investor-state dispute procedures and treaty expiration.

Trump has long been a critic of NAFTA. Earlier this month, in a White House meeting with governors and members of Congress, he labeled the treaty “a disaster” and suggested that he might terminate the deal before concluding, “I’m not going to do that.”

The talks are continuing, with Canada and Mexico receiving a temporary exemption from Trump’s import tariffs of 25 percent on steel and 10 percent on aluminum. Those are scheduled to expire May 1 but could be extended if the talks are progressing.

Trump also lashed out at Democrats on Twitter on Monday about support for “sanctuary cities,” which offer protection to undocumented immigrants.

And he said he had instructed his homeland security secretary not to allow reported caravans of people traveling from Central America through Mexico to be admitted to the United States.

“It is a disgrace,” Trump said. “We are the only Country in the World so naive!”