The government said its highly criticized program known colloquially as Remain in Mexico was now in effect all along the border.
Critics say the move makes it nearly impossible to gain asylum in the U.S.
"At taxpayer expense, this decision will transport people seeking asylum in Arizona over 300 miles to a dangerous Mexican city, where it is difficult to access legal services and where few people have the support of their family and communities," said Katie Sharar, director of communications for the Kino Border Initiative, a Catholic organization that helps migrants.
U.S. officials say the program has helped drastically reduce the number of migrants arriving at the border.
The program "works and is a game-changer in addressing the ongoing border crisis," Acting Department of Homeland Security Secretary Chad Wolf said in a tweet Friday.
The Washington Post first reported that authorities planned on busing asylum-seekers from Arizona to El Paso. Migrants apprehended in Arizona will also be subject to the program, called Migrant Protection Protocols.
The program was launched in January and has forced more than 55,000 asylum-seekers to wait in Mexico. They often wait in squalid camps, and many are kidnapped, robbed or extorted.
Until now, the program didn't exist in the Tucson Sector, which over the past year has seen just a fraction of the migrant families who have crossed elsewhere along the border.
The Rio Grande Valley, El Paso and Yuma, Arizona, have experienced the largest volumes of migrant families, many seeking asylum.
In May, at the height of immigrant apprehensions this year, the El Paso Sector saw nearly 30,000 families come through, while Tucson saw just over 1,700.
The numbers dropped quickly over the summer in the busiest crossing spots, the result of several policies making it nearly impossible to seek asylum in the U.S.
Tucson saw an uptick in families, to about 1,900 in September.
Immigrants and advocates say the program violates basic rights of asylum-seekers and places them in grave danger by making them wait in cities where even the U.S. government warns against visiting.
A legal challenge of the program is pending in California.