TIJUANA, Mexico — The United States on Tuesday returned a Honduran asylum seeker to Mexico, marking the beginning of a sweeping new policy that forces asylum seekers to wait in Mexico as their claims are processed in American courts. Homeland Security Secretary Kirstjen Nielsen planned to visit the San Ysidro port of entry later in the day to “assess implementation” of the new approach, according to a DHS statement.

The policy is one of the most dramatic changes to the American asylum system in decades — an attempt by the Trump administration to deter immigration by reducing the number of migrants who live and work in the United States while awaiting court dates.

Already the plan has raised numerous human rights concerns, not to mention a flurry of logistical questions that neither the United States nor Mexico has been able to answer. Where will the asylum seekers stay in Mexico for months or years as their cases are processed? How will Mexico ensure their safety? How will they obtain legal assistance?

Even the program’s launch has been chaotic. The Mexican Foreign Ministry initially said on Friday that it expected the first group of asylum seekers to arrive within hours at the San Ysidro port of entry near San Diego, even though the Mexican immigration agency said it had received no formal communication from the United States. There were no official U.S. guidelines until this week.

And the return of the Honduran asylum seeker Tuesday appeared to be ad hoc; he was rushed to a van by a Mexican immigration agent with no formal announcement of when ­others would follow. Rodulfo Figueroa, the national migration agency’s representative in Tijuana, said no others would be released Tuesday.

Carlos Gomez Perdomo, the first asylum seeker to be returned, was stunned Tuesday to find himself back in Mexico after starting his journey in October to reach the United States and make his case. He had sought asylum on Monday morning after spending months on a waitlist.

“I said to myself, ‘I don’t know what’s happening, and I don’t know what will happen in this case,’ ” said Gomez Perdomo, 55, who arrived as part of a migrant caravan from Honduras in November. “After I have spent so much time battling, all the time I’ve spent outside my country, to come to this place with a goal and to lose. I have not advanced at all. Instead I seem to have gone backward.”

The Mexican government has said repeatedly that it disagrees with the policy, which officials argue is being imposed on the country by the United States. Tonatiuh Guillén, the commissioner of Mexico’s immigration agency, said Monday night that Mexico would admit only Central Americans between the ages of 18 and 60 as part of the new policy. Because a large portion of asylum seekers are family units or unaccompanied minors, the Mexican response could erode the potential impact of the new policy.

In a Jan. 25 memo, Nielsen said U.S. officials had proposed the program as a “joint effort with the Government of Mexico” to develop a regional plan to deal with “irregular migration,” smuggling and trafficking.

The asylum seekers required to wait in Mexico will be given ­multiple-entry permits to attend their court dates in the United States and then return to Mexico. Some asylum seekers, including the one who arrived Tuesday, have Mexican humanitarian visas that are valid for one year. After a quick initial processing at the border, migrants will have a hearing in 45 days and a decision within a year, according to DHS.

U.S. officials said they would screen the asylum seekers to ensure that people at risk of persecution in Mexico are not made to wait across the border. But lawyers remained concerned about how the United States would make that determination. Mexico’s northern border includes some of the country’s most dangerous cities, where migrants often face extortion or kidnapping. Implementing the new U.S. policy there, human rights advocates say, is likely to have tragic consequences.

“We have worked with people who are under active threat here in Mexico, who have had to move between shelters and stay in secret locations because they are being actively pursued throughout Mexico, and throughout Tijuana. This is literally sentencing people to die,” said Nicole Ramos, an attorney with the Tijuana-based legal aid office Al Otro Lado.

The city’s resources are already stretched by the scores of migrants who have arrived in recent months. About 2,400 people are already waiting in line for asylum, a product of the U.S. system known as “metering,” or limiting the number of asylum seekers who can begin their cases each day. The first members of the newest migrant caravan arrived in Tijuana in recent days, with thousands more apparently on the way.

Some of the city’s shelters are already nearing capacity, housing a steady stream of migrants from across Mexico and Central America and others who arrive from countries around the world to make it to the U.S. border. The emergency shelter that was opened to house thousands of people who arrived from Central America in the caravan in November was scheduled to close this month.

Many of the directors of the city’s independent or church-run shelters said that through last weekend they had not been informed that they should prepare to receive returned migrants.

“They’re going to ask us at the last minute to bail them out,” the Rev. Pat Murphy, the director of the Casa del Migrante, one of the long-standing shelters in the city, said of federal officials.

Mexico’s new president, Andrés Manuel López, Obrador promised a more humane response to migration. At times, his administration appears to be delivering. When the new migrant caravan crossed into Mexico, for example, the government issued many of those migrants humanitarian visas, allowing them legal entry into the country. But Mexican officials have expressed concern about their ability to provide a comprehensive humanitarian response to the new U.S. policy.

Gustavo Magallanes Cortés, the director of migrant affairs for the Mexican state of Baja California, said Monday that he feared that federal budget cuts this year — of about $1.2 million in Baja California — to a migrant assistance fund had left small shelters scrambling for resources.

“They open the northern border, they open the southern border, but then they close the funds to help us,” Magallanes Cortés said.

For migrants who are waiting in Tijuana for U.S. asylum, rumors about the U.S. policy changes have caused fear about upcoming asylum proceedings. Karen Martinez, 28, a mother of three from San Pedro Sula, Honduras, said that she put her name on the waiting list last weekend, even after hearing about the policy, because she had just found a sponsor to receive her in the United States.

She said that she fled domestic violence and threats from her ex-husband in July, then joined the thousands of people in the caravan that headed north in October.

“It’s not my intention to stay in Mexico. A dollar is worth much more in the U.S., and I want to get as far away from my problems as possible,” she said. “But if we have to wait here, then that’s what we’ll have to do.”

Sieff reported from Mexico City. Maria Sacchetti in Washington contributed to this report.