Washington — More than 15,000 asylum seekers have been returned to Mexico by U.S. authorities to await hearings in American immigration courts as the Trump administration accelerates a controversial policy it believes can help border officials cope with an unprecedented surge of families from Central America heading towards the southern border.

Under the policy — known as "Remain in Mexico" but officially called the Migrant Protection Protocols (MPP) — 15,079 Central American migrants who claimed asylum at U.S. ports of entry along the southwestern border have been turned back to Mexican border cities as their petitions are processed in the U.S. immigration court system, a Mexican government official told CBS News Tuesday. About 5,000 of those migrants have been sent back over the past two weeks.

The latest benchmark of those returned under the Department of Homeland Security (DHS) program, which is being challenged in court, comes as the Trump administration is preparing to expand it along the entire border with Mexico. Currently, the policy is in place at ports of entry in El Paso, Calexico and San Diego — where it made its debut last December under former DHS Secretary Kirstjen Nielsen.

In this June 10, 2019, photo, Mirna Esperanza Martinez, from El Salvador, rests with family members at the Buen Pastor migrant shelter in Ciudad Juarez, Mexico. AP

According to the Mexican government official, as of Monday, 5,300 migrants have been turned back in San Diego, 3,168 in Calexico and 6,611 in El Paso. The pace of returns at El Paso is particularly noteworthy, since officials there only began implementing the policy in March, months after San Diego started.

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Under the agreement reached by the U.S. and Mexico earlier this month to avert President 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. Last month, apprehensions at the southern border hit a 13-year high.

"We were able to do this to the tune of a couple hundred people per day," Pompeo told reporters at the State Department. "We now have the capacity to do this full throttle and engage this in a way that will make a fundamental difference in the calculus for those deciding to transit Mexico to try to get into the United States."

DHS, however, has not publicly identified new ports of entry for the expansion of MPP. "DHS is reviewing all options to expand MPP quickly but deliberately across the Southwest Border, but is not in a position to confirm new locations at this time," one DHS official told CBS News.

The administration is betting that the expansion will send a powerful message of deterrence to people in Central America considering undertaking the perilous journey north. Even if they arrive at the U.S. border and seek asylum, this line of reasoning goes, they will likely be forced to wait in Mexico for months while their cases are adjudicated.

But attorneys and Democrats believe the government is not equipped to rapidly and effectively expand the policy — at least without the process being chaotic and putting thousands of asylum seekers in dangerous situations. The Mexican cities of Tijuana and Ciudad Juarez, which border San Diego and El Paso respectively, are considered to be among the top five most violent cities in the world, according to a 2018 report by a Mexican non-profit that ranked cities based on their homicides rates.

Before the roll out of "Remain in Mexico," asylum seekers who arrived at the three designated ports of entry were allowed to stay in the U.S. to wait for their immigration court appearances. This is still the case along the rest of the border, where migrants seeking asylum are housed in detention centers, temporary tents or shelters. Some are also released into the U.S. when there is no space — which Mr. Trump frequently denounces as "catch and release."

Prior to MPP's initial implementation, asylum seekers who came to the U.S. through San Diego, Calexico and El Paso had their cases distributed among immigration courts across the U.S., since some migrants were held by the government in different detention centers, and those who were released moved throughout the country and requested hearings at courts near their new homes.

But with MPP, a small number of judges in San Diego and El Paso are responsible for handling the cases of all migrants who claim asylum at these ports of entry and are sent back to Mexico, creating a bureaucratic logjam that raises questions about the feasibility of expanding the program.

The administration will likely need more immigration courts and judges to expand the policy. Since immigration courts are not independent from the Executive Branch — they are part of the Justice Department — the administration can open new ones, but the system has been dealing with years of backlog and shortage of judges.

Additionally, the implementation of Remain in Mexico is threatened by ongoing litigation. The administration had halted the policy for a few days in April after a federal judge blocked the government from continuing it. That same month, however, the Ninth Circuit Court of Appeals granted the government's emergency motion to stay the lower court's ruling, allowing DHS to resume the practice.

The Ninth Circuit has not made a final ruling, but already heard oral arguments from the government and plaintiffs, a group of immigrant rights groups and asylum seekers. They argue that migrants subjected to the policy face great risk and danger in Mexico.