US Homeland Security Secretary Kirstjen Nielsen and the Mexican government released simultaneous announcements about the plan on Thursday but said details about the rollout would be forthcoming.

Mexico's government said on Friday it wanted more details from the United States on the plan, and vowed not to deport people seeking refuge. It is not clear how Mexico plans to house what could be thousands of people from Central America for the months, or years, it takes US immigration cases to be heard. There is currently a backlog of more than 800,000 cases pending in immigration courts.

"Today, I'm going to ask the US authorities to give us many details," said Mexico's foreign minister, Marcelo Ebrard, adding that the country would lay out its position more clearly on Monday once it had more information.

The immediate lack of guidance has also left US immigration judges, who will have to process the bulk of the asylum claims through their courts, in the dark, said Ashley Tabaddor, the head of the immigration judges' union.

"Information certainly hasn't been shared with the people who may be responsible for having to implement this," said Tabaddor. "We haven't heard anything. Every peep on this we have heard through the media."

She said that has left the nation's nearly 400 immigration judges with many questions, including how the cases will be distributed among the more than 60 immigration courts.

Currently asylum seekers that have been released into the interior of the United States are assigned to courts near where they live. Tabaddor said it was not clear if judges would now be expected to travel to border courts en masse to hear cases there.

Kathryn Mattingly, a spokeswoman for the Executive Office for Immigration Review, which oversees the immigration courts, said the asylum hearings would be conducted "in accordance with applicable law." But she referred questions about logistics to the US Department of Homeland Security, which did not immediately respond to requests for comment.

Before asylum cases get a hearing in immigration court, migrants have to pass an interview with an official from US Citizenship and Immigration Services, or USCIS, to determine whether they have a "credible fear" of returning to their home country.

One asylum officer tasked with conducting those interviews told Reuters, on condition of anonymity, that they "didn't even know" about the new policy until after it was announced.

"The fact that we didn't have a clue is pretty true," the officer said. A spokesman for USCIS also referred questions to DHS, as did a spokesman for US Customs and Border Protection.

MEXICAN CONCESSIONS

The accord was widely viewed as a concession by Mexico's new president, Andres Manuel Lopez Obrador, to US President Donald Trump, who has threatened to shut down the Mexico-US border if the flow of migrants is not contained.

It is unclear how many migrants the new policy could end up returning to Mexico, and Ebrard said he did not believe the measure could be applied retroactively.

Local officials in Mexican border states are worried they will not be able to deal with the influx, said Rodolfo Olimpio, head of Baja California's state council on migrant assistance

"It is clear the Mexican Federal Government does not have the infrastructure, capacity or legal framework for this to happen. And the state and municipal governments of border states also lack the budget and conditions for this," Olimpio said. He said recent budget cuts have also left migrant shelters strapped for cash.

Serious doubts remain over whether Mexico can keep vulnerable asylum seekers safe. Authorities are investigating the deaths of two Honduran teenagers kidnapped and killed in the border city of Tijuana last weekend.

LEGAL CHALLENGES

To send people to Mexico, the Trump administration is invoking a section of the Immigration and Nationality Act that allows the government to return migrants to a foreign country bordering the United States pending their immigration process. Immigration advocates are expected to file lawsuits challenging the plan, but any legal arguments could turn on the details of how the policy will be implemented.

"Right now, we don’t know all the logistics, and I suspect the people on the ground who work for the government don't know all the details," said Lee Gelernt, an attorney from the American Civil Liberties Union who has led several efforts to block the Trump administration's immigration policy changes in court. "From what we know now, we believe it cannot be implemented lawfully."

Immigration advocates raised concerns about the difficulty accessing lawyers to help navigate complex asylum claims.

Almost all communication from immigration courts comes through the mail and the Mexican postal system is notoriously dysfunctional.

"I don’t understand, if people are living in one of these shelters in Tijuana, how they would get mail," said Kara Lynum, an immigration attorney based in Minnesota who recently volunteered in Tijuana to help asylum seekers.

Mexico has pledged to provide work visas to migrants and Deputy Interior Minister Alejandro Encinas said the government's public works plans in the south of the country could attract labourers.

Ebrard reiterated that Mexico was not planning on becoming a "safe third country," which would oblige those seeking asylum who arrive first in Mexico to apply for asylum there, saying "We haven't signed a deal, we're not going to."