Border Patrol, a CBP agency that handles law enforcement between ports of entry, said it has returned to Nogales 18 migrants who crossed the border on Thursday. Their immigration court dates were set in El Paso, Texas, more than 300 miles away.

Since late November, more than 600 asylum seekers crossing the border near Tucson, Ariz., were transported by U.S. authorities on buses to El Paso, where they were then sent just across the border to Ciudad Juárez, Mexico, and assigned court dates often months in the future.

Now, the migrants returned to Nogales must arrange their own transportation to attend asylum hearings. On the Mexican side of the border, the journey from Nogales to Juárez takes significantly longer—about 371 miles, or nearly eight hours by car, according to Google Maps, about three hours longer than the drive takes on the U.S. side.

The change will require migrants to traverse one of the most dangerous regions of Mexico, along the border between the states of Sonora and Chihuahua, where rival gangs are engaged in a turf battle. Nine U.S. citizens, members of a Mormon family living in the area, were killed in an ambush on a Sonora highway in November, straining relations between Mexico and the U.S. Under State Department guidance, U.S. government employees aren’t permitted to travel through the area.

Two other border areas assign hearings to migrants at different ports of entry from where they are returned, but the distances are only about one-third that of the new Arizona requirement. Migrants crossing near Calexico, Calif., are returned to Mexicali, Mexico, and asked to appear in court in San Diego, an approximately 110-mile journey. Migrants crossing near Eagle Pass, Texas, are sent to Piedras Negras, Mexico, and required to appear at the tent court in Laredo, Texas, about the same distance away.

“This is a new level of logistical impossibility being put in front of people already facing just a litany of due-process violations preventing them from following through with their asylum claims,” said Ian Philabaum, an attorney with the Innovation Law Lab, which is suing the Trump administration over the legality of Remain in Mexico. A decision in the case from the Ninth Circuit Court of Appeals is expected soon.

Roy Villareal, the Tucson sector chief for U.S. Border Patrol said the change would save the agency money since it ends busing to Texas and shortens an asylum seeker’s stay in custody. It would also free up his agents to return to the work of apprehending migrants.

“Operationally, the more agents I can get back into enforcement roles, the more impact that will have,” he said.

Asked whether the long journey through dangerous Mexican territory might deter some migrants from attending their immigration court hearings, Mr. Villareal said he expects to be criticized for any new policy change, regardless of the outcome, and added that deterring migrants was the intent.

“First and foremost, [the return to Nogales] should serve as a deterrent,” he said.

Until November, asylum seekers in Arizona were some of the last admitted into the U.S., rather than returned to Mexico. Most migrants seeking asylum arrive in the U.S. as families. A court settlement prohibits the government from detaining families for longer than 20 days.

As news spread that migrants crossing the Arizona border weren’t being sent back to Mexico, the Tucson border sector became the second-busiest crossing point in recent months, trailing only the Rio Grande Valley. In October and November 4,674 migrants crossed the border illegally near Tucson, more than double the number during the same two months the previous year. Border crossings plummeted in most other areas of the border over the same period.

The El Paso court already handles the largest Remain in Mexico docket, with more than 16,000 such cases filed there, of 56,000 across the country, according to the Transactional Records Access Clearinghouse at Syracuse University, which tracks immigration court data.

The expansion of Remain in Mexico furthers the Trump administration’s goal of reducing access to the nation’s asylum system, which administration officials say immigrants have exploited to gain access to the U.S., where they can work while their cases make their way through the courts.

Asylum is a legal protection that anyone can seek if they are fleeing political, religious or other persecution in their home countries. Though crossing the border without permission is illegal, U.S. law allows foreigners to apply for asylum no matter how they entered the country.

“The Department is fully committed to the program and will continually work with the government of Mexico to expand and strengthen it,” acting Homeland Security Secretary Chad Wolf said in a statement. Mexico’s embassy in Washington didn’t respond to a request for comment.

Migrants returned to Mexico are never released into the U.S. They attend court hearings in CBP custody, and even migrants who have won asylum or another form of relief allowing them to remain in the U.S. have been routinely sent to Mexico pending a government appeal of their cases.

Thousands of people have been waiting months in Nogales for their asylum hearings. Immigrant advocates say the city’s few shelters have long been over capacity and many migrants have rented hotels or rooms in private houses.

Joanna Williams, director of education and advocacy for the Kino Border Initiative, a binational immigrant aid group in Nogales, said she is concerned about how quickly the new policy was launched. She said Mexican officials have advised that as many as 30 people could be returned daily, a figure that could quickly overwhelm the limited resources for assistance.

“We’re concerned that already vulnerable people will be homeless,” Ms. Williams said.

Corrections & Amplifications
More than 600 asylum seekers crossing the border near Tucson, Ariz., were transported El Paso, Texas, since late November. An earlier version of this article incorrectly said more than 1,600 asylum seekers were transported. A photo caption earlier misidentified a Border Patrol vehicle as belonging to Customs and Border Protection. (Jan. 2, 2020)

Write to Michelle Hackman at Michelle.Hackman@wsj.com and Alicia A. Caldwell at Alicia.Caldwell@wsj.com