U.S. Customs and Border Protection on Sunday shut down one of the largest land border crossings between California and Mexico for hours, as hundreds of migrants reportedly tried to storm past police officers to cross the border.

Hundreds of migrants—many of whom had earlier staged a peaceful march calling on the Trump administration to hear their asylum claims—were reportedly enveloped in tear gas after U.S. agents sprayed several rounds of it toward people trying to breach the fence at the San Ysidro entry point, according to the Associated Press. While the tear gas was reportedly aimed at migrants trying to penetrate parts of the fence on the Mexican side of the border, witnesses reported seeing the gas fan out to reach migrants nowhere near the fence, including children who ran away from the area coughing and screaming.

The San Diego office of U.S. Border Patrol tweeted shortly after news of the chaos broke that pedestrian crossings at the San Ysidro Port of Entry had been suspended, and that vehicular traffic in both directions had been halted. The entry point was reopened several hours later.

The facility is one of the busiest border crossings in the world, with 20,000 northbound pedestrians entering the United States every day, primarily for work. More than 5,000 migrants, primarily from Central America, have taken up residence at a nearby sports complex after arriving in Tijuana as part of a larger caravan of migrants.

The crossing has been the scene of growing unrest recently as migrants stuck on the southern side of the border have clashed with both Mexican police and U.S. immigration agents out of frustration with a backlog in processing asylum claims.

Even as tensions peaked at the border Sunday, President Trump seemed determined to make the influx of migrants, many of them asylum seekers, the responsibility of Mexico and Mexico alone.

“Would be very SMART if Mexico would stop the Caravans long before they get to our Southern Border, or if originating countries would not let them form (it is a way they get certain people out of their country and dump in U.S. No longer).” he tweeted.

Less than a day earlier, he took to Twitter to declare that asylum seekers arriving at the U.S. southern border would “not be allowed into the United States until their claims are individually approved in court.” That proclamation appeared to be yet another instance of Trump assuming he will get what he wants when it comes to immigration policy, as the plan he referenced hinges on a deal with the incoming government of Mexico’s president-elect, Andrés Manuel López Obrador, that Trump has reportedly not yet struck.

Immigration advocacy groups and civil-rights organizations that have mounted numerous legal challenges against Trump’s anti-immigration apparatus say that they’re likely to take this latest plan to court as well—when and if it ever actually materializes.

“We have not seen a specific proposal, but any policy leaving asylum seekers stranded in Mexico will put them in danger,” said Lee Gelernt, the deputy director of the American Civil Liberties Union’s Immigrants’ Rights Project. “The Trump administration should focus on providing a fair and lawful asylum process in the U.S. rather than seeking more ways to undermine it.”

As The Washington Post first reported on Saturday, the deal—named “Remain In Mexico”—would require almost all individuals seeking asylum at the U.S. southern border to, as the name suggests, remain in Mexico while their asylum claims are processed and adjudicated, regardless of their nation of origin. Only those who can prove a credible fear of violence in Mexico, under the Trump administration’s increasingly narrow definition of “violence,” will be allowed to pass through what amounts to an invisible wall on the U.S. border while their asylum claims are evaluated.

“If you are not determined to have a reasonable fear of remaining in Mexico, you will remain in Mexico,” a Department of Homeland Security memo obtained by the Post reportedly instructs asylum officers to tell asylum seekers.

Olga Sánchez Cordero, a member of López Obrador’s incoming administration, initially confirmed the plan as going forward once he takes power on December 1, calling the proposal a “short-term solution.” But Sánchez Cordero denied that any plan had been formalized hours later, issuing a statement that “There is no agreement of any sort between the incoming Mexican government and the U.S. government,” noting that a plan for Mexico to indefinitely host migrants seeking asylum in the United States had been “ruled out.”

The plan would be in direct contradiction to the foreign policy platform that helped sweep López Obrador into power. The former mayor of Mexico City, López Obrador has taken firm stances against President Trump’s proposed 2,000-mile border wall along the U.S.-Mexico border, and called on the administration of outgoing President Enrique Peña Nieto to sue the United States for human rights violations resulting from President Trump’s immigration policy. He has also asked for solidarity with a caravan of Central American migrants fleeing violence and persecution in the so-called “Northern Triangle” countries of El Salvador, Guatemala and Honduras.

“The Americans want us to put [immigration centers for migrants] on the southern border with Guatemala, so that we will do their dirty work for them,” López Obrador told The New Yorker before his election. “No, we’ll put it here, so we can look after our immigrants.”

But the growing number of migrants camped out at U.S. ports of entry while they await processing has strained both resources and patience for border communities in Mexico. Juan Manuel Gastélum, the mayor of Tijuana whose harsh criticism of migrants has won praise from President Trump, has declared a humanitarian crisis in the city, asking for U.N. intervention to help aid the roughly 5,000 migrants who have arrived in the city in recent weeks.

The Department of Homeland Security did not respond to requests for comment regarding the plan’s existence, or whether President Trump had jumped the gun in declaring the new policy by Twitter—as he has done before—although Acting Homeland Security Under Secretary for Policy James McCament said in a statement that the United States has been negotiating with Mexico “to identify and address shared issues of concern,” which “include our joint desire to promote beneficial legitimate trade and travel, interest in ensuring that those traveling to our borders do so safely and orderly, concern for the safety and security of vulnerable migrant populations.”

In a statement, White House Deputy Press Secretary Hogan Gidley sidestepped acknowledging the plan, but said that “President Trump has developed a strong relationship with the incoming López Obrador Administration, and we look forward to working with them on a wide range of issues.”

The plan would officially codify the Frankensteinian patchwork of unofficial Trump administration policies regarding asylum seekers, which have sought to severely limit the eligibility of would-be asylees while also creating a bottleneck at ports of entry that has led to migrants living on bridges as they await “more room” at asylum processing centers.

In a statement, U.S. Customs and Border Protection said that its facilities are “not designed to hold hundreds of people at a time who may be seeking asylum,” meaning that “we’ve had to limit the number of people we can bring in for processing at a given time.”

After the president’s proclamation barring migrants who enter the country illegally from seeking asylum—which has since been blocked by a federal judge—the Department of Homeland Security assured reporters that it would provide additional resources at ports of entry in anticipation of groups arriving at the border. As The Daily Beast has reported from the border, those resources have not yet resulted in faster lines for processing asylum claims, where some migrants are sleeping outside for more than a week in hopes of being allowed to pursue their claims.

If the “Remain In Mexico” plan does not end up materializing, however, President Trump has hinted at an even more extreme proposal: shutting down all entry into the United States, period.

“If for any reason it becomes necessary, we will CLOSE our Southern Border,” the president tweeted on Saturday. “There is no way that the United States will, after decades of abuse, put up with this costly and dangerous situation anymore!”