WASHINGTON — The Trump administration said Thursday that it would start blocking a small number of asylum-seekers from entering the United States from Mexico, using the San Ysidro border crossing near San Diego as the first location to turn back immigrants applying for refugee status.

The policy to block asylum-seekers was first announced last month by Kirstjen Nielsen, the homeland security secretary, and Secretary of State Mike Pompeo. It will gradually be expanded over the next two weeks at border crossings with heavy foot traffic in Texas, New Mexico and Arizona, according to a senior U.S. official briefed on the move, who spoke on the condition of anonymity.

The move is intended to dissuade immigrants, mostly from Central America, from making the long and dangerous journey through Mexico to the southwestern United States border. The policy is likely to intensify pressure on Mexican authorities, who are already struggling to deal with thousands of Central American immigrants who have applied for humanitarian visas in Tapachula, near the border with Guatemala.

It will apply both to some asylum-seekers who try to enter the United States at border crossings and to people who are stopped while illegally trying to enter. Asylum-seekers generally have been allowed to wait in the United States, often for years, for their cases to be processed.

“For far too long, our immigration system has been exploited by smugglers, traffickers and those with no legal right to be in the United States,” said Nielsen, who described the new protocol as a “methodical common-sense approach” in a statement late Thursday.

While she cited the danger posed by adults and criminals, a fact sheet explaining the justification for the new policy, described as “migrant protection protocols,” cited an unmanageable influx of children and families from Central America as the main reason for the change.

The protocol was necessary to counter “misguided court decisions and outdated laws that have made it easier for illegal aliens to enter and remain in the U.S.,” Nielsen said. Under the provisions of the Immigration and Nationality Act of 1965, the new policy can be enforced immediately without congressional approval, she added.

Despite President Donald Trump’s efforts to limit refugees, the number of migrants who asked for asylum last year out of fear of returning to their home countries jumped nearly 70 percent from 2017, according to Department of Homeland Security data. Nearly 60 percent of all foreigners asking for asylum were people in families.

The announcement was made Thursday amid tense negotiations between U.S. and Mexican officials over how the policy would be carried out. It was primarily intended to relieve pressure on U.S. immigration officials, but the senior official said it was also intended to highlight what the White House has called a “humanitarian crisis.”

Trump shut down the government over a month ago over demands that House Democrats allocate $5.7 billion for the construction of a wall along the southwestern border. His plan failed in the Republican-controlled Senate on Thursday, leaving no clear path for an end to the longest funding lapse in U.S. history.

Under the new policy, asylum-seekers who are turned back to Mexico will receive a notice informing them of their right to be heard in U.S. immigration court after 45 days, officials said.

Immigrants rights groups are expected to challenge the move in court, and they are likely to argue that forcing asylum-seekers to wait in Mexican border cities puts them at greater risk in a region known for its high murder rates.

Mexican authorities had long expected the move, but were taken aback when they were told early Thursday that the plan would take effect Friday.

That will force the Mexican government to create a system for processing, housing and protecting the asylum-seekers who would now congregate on their side of the border.

This article originally appeared in The New York Times.