Washington — The Trump administration on Monday touted the sixth consecutive monthly drop in apprehensions of migrants along the U.S.-Mexico border, saying its stringent measures to restrict access to the U.S. asylum system have sent a powerful message deterring would-be migrants from Central America and around the world.

U.S. authorities apprehended about 21,000 adults, 9,000 families with children and more than 3,000 unaccompanied minors in November, a significant decline from the more 133,000 migrants encountered in May, when an unprecedented surge of U.S.-bound Central American families peaked and overwhelmed detention centers along the border.

"They will no longer be allowed to exploit our laws and be allowed into our country based on fraudulent claims or because they arrive here with a child," Acting Customs and Border Protection Commissioner Mark Morgan said at a press conference Monday. "Those loopholes have been closed."

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"Remain home" is the message the administration wants to deliver to families considering the trek north, Morgan said. For asylum-seekers with "legitimate" claims, he added, the message is to go to first neighboring country for "immediate relief." For adults, the message is even more stern.

Acting Customs and Border Protection Commissioner Mark Morgan holds a press conference in Washington on Monday, December 9, 2019. CBS News

"If you are a single adult who has illegally entered our country, we are going to detain you and make every effort to prosecute you for violating our sovereign laws," he said.

The cornerstone of the administration's efforts to deter migrants along the border is the so-called "Remain in Mexico" policy, which has required more than 54,000 asylum-seekers to wait in Mexico for the duration of their immigration court proceedings. The program, which is being challenged in court, has spurred the creation of encampments and shelters in northern Mexican cities where conditions are often squalid and life is uncertain for tens of thousands of migrants.

Many of the places where the U.S. is returning migrants subject to the policy are plagued by rampant insecurity, including two cities in the state of Tamaulipas, where the State Department warns Americans not to travel because of widespread violent crime, gang activity and extortion.

The Mexican National Guard patrols an encampment in Matamoros, Mexico, on December 7, 2019. VERONICA CARDENAS / REUTERS

But Morgan hailed the policy and suggested that officials could expand the program to include so-called "extra-continental" asylum-seekers, such as migrants from non-Spanish-speaking countries like Brazil and nations in sub-saharan Africa. Morgan did not provide a specific timeline, but said the need is "urgent." Currently, U.S. officials generally only return non-Mexican migrants from Spanish-speaking countries to Mexico.

In addition to "Remain in Mexico," U.S. officials have been implementing a pilot program along the El Paso sector of southern border designed to fast-track the processing and deportation of asylum-seekers subject to a sweeping regulation allowed by the Supreme Court which renders most non-Mexican migrants ineligible for asylum.

The administration has also secured bilateral agreements with Guatemala, Honduras and El Salvador to reroute asylum-seekers to these countries. So far, only the deal with Guatemala has been implemented, with the U.S. sending the first asylum-seeker subject to it there last month.

Texas Congressman Joaquin Castro, chair of the Congressional Hispanic Caucus, said the drop in border apprehensions has come "at the expense of humanity," arguing at a press conference on Tuesday that the administration has tried to "get rid of the problem without solving it."

"They've placed people in dangerous Mexican cities that the United States government advises its own people, us, not to visit," Castro said when asked about the matter by CBS News.

"I would encourage the president to go view for himself the consequences of his policies, and go to Matamoros and go to Juárez, Mexican cities along the border, and see what's happening to the people that the United States has refused, under this administration, to treat humanely as asylum-seekers," he added.