The Trump administration will start returning non-Mexican migrants who claim asylum in Texas' Rio Grande Valley back to Mexico under the second expansion of the controversial "Remain in Mexico" policy.

U.S. officials are slated to make the first returns of migrants who claim asylum in the Texas border city of Brownsville "as early as" Friday, a Department of Homeland Security (DHS) official told CBS News.

The move means asylum seekers will now be sent to Matamoros, the second largest city in Tamaulipas, one of five Mexican states the State Department warns Americans travelers not to visit because of widespread crime and the risk of being kidnapped.

Get Breaking News Delivered to Your Inbox

"Violent crime, such as murder, armed robbery, carjacking, kidnapping, extortion, and sexual assault, is common. Gang activity, including gun battles and blockades, is widespread," the State Department says in its travel advisory of Tamaulipas.'

People cross the the Brownsville-Matamoros International Bridge on the U.S.-Mexico border in Matamoros, Tamaulipas, Mexico June 28, 2019. CARLOS JASSO / REUTERS

Just this Thursday, Mexico's ambassador to the U.S., Martha Bárcena, said the Mexican government was not prepared for the expansion of "Remain in Mexico" in Tamaulipas."We recognize there are certain areas of Mexico in which the challenges of security are higher," Bárcena said at a CQ Roll Call event. "So, that is why we've been very careful of not opening up, for example, the returns in Tamaulipas."

Many asylum seekers from Central America and other parts of the world have already been forced to wait in Matamoros because of a "metering" policy meant to slow the flow of migrants by having them put their names on a list and waiting in Mexico for their turn to request asylum in the U.S.

Migrants from Central America, the Caribbean, South America, and Africa line up for a meal donated by volunteers from the U.S., at the foot of the Puerta Mexico bridge that crosses to Brownsville, Texas, in downtown Matamoros, Tamaulipas state, Mexico, Wednesday, June 26, 2019. Hundreds of migrants, some of whom have been in line for months, are awaiting their turn to request asylum in the U.S. Rebecca Blackwell / AP

The expansion of the policy, officially called the Migrant Protection Protocols (MPP) program, to Brownsville also represents the practice's first implementation in the Rio Grande Valley, the most heavily patrolled sector along the U.S.-Mexico border.

The first expansion occurred in Laredo, Texas. The policy is also in place at ports of entry in El Paso, Calexico and San Diego — where it made its debut last December.

Nearly 20,000 asylum seekers have returned to wait in Mexico, according to figures by the Mexican government.

Migrant children play soccer as their asylum-seeking families wait in hopes of being let through the nearby U.S. port of entry at a makeshift migrant camp near the Gateway International Bridge in Matamoros, Tamaulipas, Mexico, June 26, 2019. LOREN ELLIOTT / REUTERS

The "Remain in Mexico" program faces court challenges, and has been strongly criticized by immigrant advocates, Democrats and even some of the asylum officers overseeing it. They believe the policy violates U.S. and international refugee law because it places desperate asylum seekers at risk in Mexico's border cites — many of which are plagued by crime and violence.

While they wait for their day in a U.S. court, these Central American migrants struggle to find shelter and employment in Mexico. Some face persecution and extortion, and most will show up to court without a lawyer. Lawyers and organizations have also struggled to help and represent the tens of thousands of migrants who have been returned under the program.

Under the agreement reached by the U.S. and Mexico last month to avert Mr. Trump's threats to impose tariffs on Mexican goods, the Trump administration pledged to "immediately" carry out the expansion of "Remain in Mexico" along the entire southern border.

After the deal was brokered, Secretary of State Mike Pompeo called the "full-blown" expansion of MPP a "big deal" in the government's efforts to curb the flow of migration from Central American countries. In May, apprehensions at the southern border hit a 13-year high.

Although there was a dramatic drop last month in southern border apprehensions — attributed by officials mainly to Mexico's amped up immigration enforcement and the summer heat — U.S. border officers are still on track to make around 1 million arrests this year.