WASHINGTON — A day after the U.S. Supreme Court allowed the Trump administration to proceed with a policy to curb asylum claims along the Mexico border, U.S. officials said they are implementing the new rules nationwide, while analysts and migrant advocates warned it would all but slam the door on those seeking refuge in the United States.

Under the policy, migrants who decline to apply for asylum in another country while traveling en route to the U.S. border will be rendered ineligible for U.S. asylum protections and referred for rapid deportation.

U.S. Citizenship and Immigration Services, which runs the asylum program, said Thursday that the restrictions will apply broadly to anyone "who entered, attempted to enter or arrived in the United States across the southern border on or after July 16, 2019," if that person did not seek protection in another country while traveling to the United States.

Jessica Collins, a spokesperson for USCIS, said her agency's goal is to deter migrants who have tried to take advantage of U.S. humanitarian programs to gain entry at the border.

"Until Congress can act with durable, lasting solutions, the rule will help reduce a major 'pull' factor driving irregular migration to the United States and enable the administration to more quickly and efficiently process cases originating from the southern border, leading to fewer individuals transiting through Mexico on a dangerous journey," Collins said in a statement.

USCIS did not respond to inquiries about how the policy will be implemented on the ground. Several analysts said it is likely to have a major effect on other Trump administration programs aimed at deterring migrants, including the "Migrant Protection Protocols" that have led to more than 42,000 asylum seekers being returned to Mexico this year to wait outside U.S. territory as their claims are processed.

Those who presented themselves at the U.S. border after the policy was first implemented in mid-July but then returned to Mexico would probably face deportation now if they try to enter the United States for their court hearings, potentially leaving thousands of applicants in limbo on the Mexican side.

During the past 11 months, the number of migrants arriving at the U.S. southern border has approached 1 million, nearly double the 2018 total. The influx includes a record number of migrants traveling as part of family groups, border-crossers who typically express a fear of harm if deported. That fear typically precipitates an interview with a U.S. asylum officer.

Asylum seekers whose claims are determined to be credible usually are referred to the U.S. immigration court system for a more extensive hearing, but in recent years, the number of pending asylum cases has soared to more than 330,000, according to USCIS officials.

Under the new rules, asylum applicants who traveled through Mexico but did not apply for safe refuge there would be denied access to U.S. courts and deported to their home countries instead.

"This is a big deal," said Doris Meissner, a senior fellow at the Migration Policy Institute, a Washington think tank, who ran the U.S. immigration system during the Bill Clinton administration.

"Its broad purpose is to further reduce and almost eliminate the ability to apply in the United States for asylum," she said. "For people that have serious claims for asylum, their only practical alternative is filing in Mexico."

Mexico's asylum program already is buckling under a surge in new applications, and it remains overwhelmed and underfunded, observers say. Mexican Foreign Minister Marcelo Ebrard — who met with Vice President Mike Pence, Secretary of State Mike Pompeo and other senior Trump officials for migration talks in Washington on Tuesday — called the court ruling "unprecedented," amid concerns that tens of thousands of migrants could stall their journeys in northern Mexico once they realize they are likely to be deported from the United States if they make asylum claims.

"This will squeeze migrants into remaining in Mexico and gives Trump carte-blanche to deny asylum," said Tony Payan, director of the Center for the United States and Mexico at Rice University.

Payan predicted the court ruling would scramble existing migration patterns and probably prompt more people to pay smugglers to help them make a risky illegal crossing.

"These kinds of decisions are subject to a balloon effect," he said. "If you deny them entry at ports of entry, then they are going to try to come through between the ports and increasingly rely on smugglers and organized crime."

One asylum officer, who spoke on the condition of anonymity because the person was not authorized to discuss the policy with reporters, said the policy was likely to produce friction between officers and their supervisors.

Ken Cuccinelli, the acting head of the USCIS, immediately hailed the court ruling on Twitter as a "YUGE Win" (sic) for Trump.

Asylum officers would still have some discretion to let applicants into a process that could allow them to stay in the United States: Officers who believe an applicant faces true peril if sent home would still be able to refer that person to U.S. immigration courts if they determine they might qualify for a lesser status known as "withholding of removal." Withholding is not a path to legal U.S. residency, but it stops the deportation process and would potentially give the person more time to pursue other legal claims on U.S. soil.

Officers also could refer asylum seekers for withholding under the U.N. Convention Against Torture if the claimant can demonstrate that they are likely to be harmed with the consent or participation of their home country's authorities.

It is more difficult to qualify for those forms of protection, the officer said, because an applicant must demonstrate a higher likelihood that they will face harm: "Time will tell once asylum officers and their supervisors figure this out and get more comfortable with what should be 'reasonable possibility' " of harm.

George Mason University professor Guadalupe Correa-Cabrera said she spent the summer traveling the border region and surveying migrants stuck on the Mexican side of the international line. Many of the migrants she interviewed said they had not applied for asylum in Mexico because they did not know how to do so. They said they also were not interested in settling anywhere other than the United States.

"They don't consider Mexico to be a safe place," Correa-Cabrera said.

Immigration attorney Kara Lynum said that during a recent trip to the Mexican city of Juarez, she spoke to migrants who were grabbed in the streets, received threatening text messages from kidnappers and had watched black cars circling the shelters where migrants are awaiting their chance to cross into the United States. In one case, a woman's alleged kidnapper entered her bedroom at a migrant shelter, Lynum said.

"I don't know that this will have a deterrent effect because the conditions that caused people to migrate still exist, and they've not been given a better option," Lynum said. "People are going to go into more remote places and more dangerous conditions to reach the United States."

The potential dangers place a significant burden on U.S. asylum officers who interview migrants seeking asylum, said Michael Knowles, spokesman for the union representing asylum officers and other USCIS employees. The union filed a brief in federal court saying asylum restrictions go against the long-held belief that the United States should be a safe haven for the oppressed. He said officers have reported lying awake at night, in tears and feeling morally compromised by the changes the Trump administration has implemented. Many career asylum officers have left the job altogether, he said.

"If the intent is to deter the meritless asylum seekers, it also has the effect of deterring bona-fide refugees for whom the system was designed," Knowles said. "There's no explanation for that."

Knowles said that USCIS supervisors cannot overrule an officer's decision to refer an asylum seeker to court for a hearing unless the officer applied the standards incorrectly. Supervisors also cannot unilaterally strike a referral simply because they do not agree with a determination, he said.

Unaccompanied migrant children still benefit from greater legal protections than adult migrants. They also would be barred from seeking asylum under the new policy, and the Trump administration is seeking to overhaul the laws that make it significantly more difficult for the government to deport children.