In break with past policy, US tells first asylum seeker to wait in Mexico The new Trump administration policy faces a likely court challenge.

The U.S. has turned back its first asylum seeker from the southern border, as part of the Trump administration's new policy requiring people to wait in Mexico for their cases to work their way through immigration courts, the Department of Homeland Security said Tuesday.

Agency spokeswoman Katie Waldman said the person seeking asylum was turned back to Tijuana at the San Ysidro port of entry in San Diego.

The move was a break with longstanding U.S. policy that allows migrants to wait inside the U.S. as their case winds through the immigration courts, a process that can take months or years.

Immigration advocates have vowed to challenge the policy in court.

"We can't send people into danger and pretend we don't have any responsibility," said Judy Rabinovitz, deputy director of the immigrants' rights project at the American Civil Liberties Union, adding that the group was trying to track down the migrant who was returned to Mexico.

Another complication is Mexico's willingness to accept the migrants. Most of the migrants seeking to cross the U.S. border are from Central American countries and not Mexican citizens.

Dualina Chavez, a spokeswoman for the Mexico embassy in Washington, told ABC News in an email on Tuesday that "Mexico won't extend the policy beyond a single border crossing, the El Chaparral crossing in Tijuana."

Waldman disputed this assertion, which was reported earlier by The Associated Press, insisting that the U.S. has the legal authority to roll out the policy along the entire U.S.-Mexico border.

"DHS has said all along it intends to expand (the new policy) across the border in an orderly, phased approach based on capacity and in coordination with our Mexican counterparts," she said.

Waldman declined to say how fast DHS plans to implement the policy outside San Ysidro, adding that U.S. officials first wanted to ensure it had the proper guidance and training in place before the changes implemented.

"We are trying to get it right the first time," she said.

The development came on the same day Homeland Security Secretary Kirstjen Nielsen visited the San Diego border crossing. Nielsen announced the new approach in a congressional hearing on Dec. 20.

ABC News' Quinn Owen contributed to this report.