WASHINGTON—U.S. Customs and Border Protection for the first time Thursday began turning around migrants seeking asylum in Arizona and sending them to Nogales, Mexico, to await U.S. court hearings that they now will need to get to on their own.

The move expands the controversial Migrant Protection Protocols or “Remain in Mexico” program, which the U.S. adopted a year ago to deal with a surge of Central American migrants at the southern border. Immigrant and human-rights advocates have criticized the policy for effectively requiring migrants to live in dangerous Mexican border cities, often for months, where the U.S. warns its own citizens to avoid traveling.

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.