US-bound asylum seekers are not yet being sent back to additional Mexican border towns to await processing after a deal on Friday called for the program to be immediately extended to the whole border, three Mexican officials said.

The text of the deal stated that the expansion of the so-called Migrant Protection Protocols (MPP), also known as "Remain in Mexico," would begin right away.

Officials in the Mexican border states of Sonora, south of the US states of Arizona and New Mexico, and Tamaulipas, south of Texas, said on Sunday, however, that they were unaware of expansion plans.

"As of today, we haven't been informed of anything," said ­Ricardo Calderon, an official with Tamaulipas' state migrant institute. 

Sonora Governor Claudia Pavlovich said on Saturday that she was also awaiting more information on the plans.

The officials did not say whether US authorities had already requested the program be extended to particular cities. Since late January, the mostly Central American asylum ­seekers are sent back across the San Diego and Calexico crossings in California, and El Paso, Texas.

Trump defended the agreement on Sunday against criticism that there were no major new commitments to stem a flow of mostly Central American migrants crossing into the US, many seeking asylum protections, and pledged to provide additional details soon.

The deal, approved by both countries, removed the threat of punishing tariffs on all Mexican exports that was issued by US President Donald Trump on May 30.

Under the agreement's terms, the Mexican government agreed to better police its southern border with Guatemala and take in possibly tens of thousands of people seeking asylum in the United States while their cases are adjudicated.

Mexico's federal migration institute had also not been notified when the expansion will be implemented at additional border crossings, according to a source at the agency who was not authorized to speak ­publicly.

The office of Mexico's foreign minister, who led the Mexican negotiating team in the talks with US officials, did not immediately respond to a request for comment.

Some Mexican border officials have warned that they do not have the resources to handle a new surge of asylum seekers.