MATAMOROS, Mexico — The United States government has sent about 900 mostly Central American and Cuban migrants back to this northern Mexico border city since expanding its controversial “remain in Mexico” program to the easternmost point on the shared border two weeks ago, Mexican authorities say.

They are among nearly 3,000 people with pending U.S. immigration proceedings who have been sent back to wait in Mexico’s Tamaulipas state, where the U.S. State Department warns Americans to avoid all travel due to high levels of violence and kidnapping.

Under the program, migrants who turn themselves over to U.S. authorities and, in most cases, request asylum, are returned to Mexico after being given a court date. To pursue their asylum cases they have to wait in Mexico.

The program began in January at Tijuana, across from San Diego, and the U.S. government has been expanding it eastward along the border, seeing it as an effective deterrent to the flood of migrants. U.S. authorities began returning migrants at the Tamaulipas city of Nuevo Laredo, on the border with Laredo, Texas, in early July.