Opponents of the Trump administration’s plan requiring all migrants seeking asylum in the United States to remain in Mexico for the duration of their immigration proceedings have vowed to challenge the policy, which they say—like nearly every other aspect of President Donald Trump’s immigration agenda—almost certainly violates constitutional protections, international treaties, and federal law.

The policy, dubbed the “Migration Protection Protocols” by the Department of Homeland Security, is “disgraceful and illegal” and “will result in the loss of life for vulnerable people seeking safety,” said Michelle Brané, director of the Migrant Rights and Justice program at the Women’s Refugee Commission. “This president has, again, chosen to exploit and endanger the lives of women and children to advance his own self-serving agenda.”

“Pushing asylum-seekers back into Mexico is absolutely illegal under U.S. immigration law,” Eleanor Acer, senior director for refugee protection at the nonprofit Human Rights First, told reporters on a conference call on Friday morning. “This scheme will increase, rather than decrease, the humanitarian debacle at the border.”

Under the proposed rule change, migrants who attempt to claim asylum in the United States at the southern border will almost universally be held in Mexico for the duration of their immigration proceedings, a process that could take years.

Calling the move “a historic measure,” the Department of Homeland Security revealed the plan on Thursday, at the same time Secretary Kirstjen Nielsen was being grilled by members of the House Judiciary Committee on the Trump administration’s numerous immigration controversies, including its family separation policy (the existence of which Nielsen denied) and the recent death of a 7-year-old migrant girl in the custody of Immigration and Customs Enforcement.

In the announcement, Nielsen said that “aliens trying to game the system to get into our country illegally will no longer be able to disappear into the United States, where many skip their court dates.” Instead, “they will wait for an immigration court decision while they are in Mexico. ‘Catch and release’ will be replaced with ‘catch and return.’ ”

Mexico’s foreign ministry, contradicting the foreign policy platform that helped sweep the country’s new president into power, said that it “will authorize, for humanitarian reasons and temporarily, the entry of certain foreign persons from the United States who have entered the country through a port of entry or who have been apprehended between ports of entry, have been interviewed by the authorities of migratory control of that country, and have received a summons to appear before an immigration judge.” (The country’s top immigration official now says that Mexico is completely unprepared to fulfill its end of the bargain.)

Organizations on the ground say that the policy is a clear violation of both federal and international law, as well as constitutional guarantees of due process—and plan to fight it in court.

“This administration knows that the border area is unsafe for women and children,” Brané said, “and still, this administration doubles down on policies that make everyone less safe.”

“The administration seems to have no plan for implementation,” said Kennji Kizuka, a senior researcher and refugee protection policy analyst at Human Rights First. “Will lawyers be able to visit their clients before hearings? Where will those hearings take place?... Access to counsel is one of the most important factors in whether or not an asylum seeker is able to live in safety in the United States.”

“ This administration knows that the border area is unsafe for women and children, and still, this administration doubles down on policies that make everyone less safe. ” — Michelle Brané, director of the Migrant Rights and Justice program at the Women’s Refugee Commission.

In addition to Article 33 of the United Nations Convention and Protocol Relating to the Status of Refugees, which prevents the forcible return of asylum-seekers to countries where they face persecution, torture or death—dubbed the principle non-refoulement in international law—advocates pointed to laws passed by Congress that mandate the admission of unaccompanied children seeking asylum at the U.S. border as being blatantly violated by the president’s policy.

“Refusing to process children very clearly violates the Trafficking Victims Protection Reauthorization Act, written specifically to protect this vulnerable population,” said Lisa Frydman, vice president for regional policy and initiatives at Kids in Need of Defense (KIND), a nonprofit that works on behalf of unaccompanied children who enter the U.S. immigration system alone. Speaking on a call with reporters, Frydman recounted interviews with unaccompanied children held in shelters in Tijuana, the conditions of which are “squalid,” Frydman said.

“Unaccompanied children are being systematically denied access to apply for protection in the United States” as they seek asylum protections, Frydman said, and their efforts to avoid both U.S. and Mexican immigration authorities are putting them in even more danger of exploitation.

Some of the children have even taken to living on the streets of Tijuana, Frydman said, where they have no access to medical treatment, food, or protection from those who might exploit them. The dangers are extreme: just this week, two Honduran children were murdered in Tijuana after being stopped by would-be robbers as they attempted to move from one shelter to another.

“All of our organizations have been on the ground in Tijuana recently and are united in our assessment that conditions there are very unstable and very unsafe,” said Wendy Young, president of KIND. Those conditions, Young continued, “are going to further deteriorate” as the number of asylum-seekers stuck at the border increases.

A 2017 study by Human Rights First documented 921 crimes against migrants committed by federal or state officials in Mexico, where nearly 70 percent of migrant children are held in “prison-like” immigration detention facilities, according to a report from Human Rights Watch, despite Mexican laws prohibiting children from being held in such facilities.

These unsafe conditions in Mexico make forcing asylum-seekers to remain their a blatant violation of the principle of non-refoulement, advocates said, and therefore a violation of international law.

“These migrant camps are not safe for children,” said Dr. Alan Shapiro, a pediatrician who co-founded Terra Firma, an organization that provides medical care to undocumented children. “They are not enclosed camps, they do not have roofs over their head.” On a recent visit to one camp in Tijuana, Dr. Shapiro said, he saw a two-year-old child who had recently suffered a seizure and had no access to medical care, or even proper food.

“This child was eating powdered baby formula out of the can—there was no water for them to mix it with,” Shapiro said.

“There are very real risks to unaccompanied children,” said Leah Chavla, a policy adviser at the Women’s Refugee Commission. “This is a system that is ripe for exploitation… Mothers that we’ve spoken with have flagged that there are a lot of new faces around the camps and they don’t necessarily feel comfortable leaving their children with strangers.”

Advocates also pointed to serious logistical hurdles for asylum-seekers to receive proper legal counsel as they navigate the labyrinthine immigration system from outside the United States, pointing to those difficulties as potential violations of due process.

“It is unclear how attorneys in the United States would be able to work in and access their clients in Mexico—if at all,” said Jennifer Podkul, senior director for policy and advocacy at KIND. “Moreover, legal services capacity in Mexico would be insufficient to address these needs or to ensure the provision of accurate legal information and preparation of cases in accordance with U.S., rather than Mexican, law.”

Those difficulties are doubled for unaccompanied children, Podkul said, in light of their age and limited ability to testify in their own defense. “Without quality legal representation, unaccompanied children and other asylum seekers will be unable to fully present their cases for protection, and as a result, may be returned to harm, danger, or death.”