President Donald Trump again threatened in a series of tweets to close the southern border next week if Mexico doesn’t “immediately” stop illegal migration into the U.S.

“The DEMOCRATS have given us the weakest immigration laws anywhere in the World. Mexico has the strongest, & they make more than $100 Billion a year on the U.S.,” Trump tweeted. “Therefore, CONGRESS MUST CHANGE OUR WEAK IMMIGRATION LAWS NOW, & Mexico must stop illegals from entering the U.S. through their country and our Southern Border. Mexico has for many years made a fortune off of the U.S., far greater than Border Costs.

“If Mexico doesn’t immediately stop ALL illegal immigration coming into the United States throug our Southern Border, I will be CLOSING the Border, or large sections of the Border, next week. This would be so easy for Mexico to do, but they just take our money and “talk.” Besides, we lose so much money with them, especially when you add in drug trafficking etc.), that the Border closing would be a good thing!”

This isn’t the first time Trump has made such threats. He also declared the situation at the border a national emergency in order to divert funds that weren’t approved by Congress to build his long-promised border wall, a wall he said Mexico would pay for countless times on the campaign trail.

Per Bloomberg:

Mexican President Andres Manuel Lopez Obrador, speaking in Spanish, told reporters that “we are going to help, we want to have a good relationship with the United States government. We are not going to enter in controversy.”

He said that some of Trump’s complaints are related to politics and “the electoral process.”

Apprehensions of undocumented immigrants spiked in February to more than 76,000, an increase of more than 39,000 compared to a year earlier, according to U.S. Customs and Border Protection. More than half were families or unaccompanied children, the agency reported.

Homeland Security Secretary Kirstjen Nielsen said in a letter to Congress sent Thursday that apprehensions will near 100,000 this month, and that her agency faces a “system-wide meltdown.” There are 4,700 migrant children in detention facilities run by Customs and Border Patrol, she wrote, calling the figure “a symptom of a broken system.”

“DHS facilities are overflowing, agents and officers are stretched too thin, and the magnitude of arriving and detained aliens has increased the risk of life-threatening incidents,” she wrote. She asked Congress for more money to build detention facilities for the migrants, and also wants the authority to rapidly deport children from Central America “if they have no legal right to stay.”

Nielsen’s agency is required to hand off most children it apprehends to shelters run by the Department of Health and Human Services, which she said in her letter is also running out of space.

A senior U.S. administration official told reporters in a conference call that the Department of Homeland Security is moving border agents from U.S. ports of entry on the border to patrol areas between them, where most illegal border crossings happen. The official said no preparations are yet being made to close the border, but that if the flow of migrants continues to increase the U.S. might “degrade” operations at ports.