Speaking as the US prepared to send the first asylum seekers back across the border, foreign ministry spokesperson Roberto Velasco said Mexico had not contributed to President Donald Trump's "Remain in Mexico" policy.
"The Mexican government disagrees with the unilateral measure implemented by the United States government. However ... we reiterate our commitment to migrants and human rights," Velasco told a news conference.
US to begin forcing asylum applicants to remain in Mexico
He said the US embassy in Mexico had informed local authorities the policy would begin within hours with the return of 20 Central American asylum seekers at the San Ysidro border crossing between San Diego, California and the Mexican city of Tijuana.
The US will continue presenting 20 people a day at that crossing, and eventually extend the policy to other points along the 3,145-kilometer (2,000-mile) border, he said.
Announced in December, the policy is meant to stop what Trump calls "catch and release" - allowing migrants who cross the border without papers and claim asylum to leave detention and remain in the United States while their cases are processed.
The US Department of Homeland Security says at least 80 percent of asylum claims are without merit - mostly by poor refugees from violence-wracked Honduras, Guatemala and El Salvador.
Many of them never show up for their court hearings, department officials say.
The system is badly backlogged, with 800,000 asylum requests currently pending.
But the "Remain in Mexico" plan has been sharply criticised by opponents on both sides of the border.
Migrants have been tortured, raped and killed in Mexico's often violent border regions, and the new policy violates their right to seek asylum in the US, activists say.
The new policy "is not only unlawful but could lead to dire and catastrophic consequences for those who are seeking to access the asylum process in the United States, a right which is enshrined in both domestic and international law," Katie Shepherd, a lawyer at the American Immigration Council, told AFP.
Trump announces deal with lawmakers to end government shutdown
Mexico said it would give returnees humanitarian visas that would allow them to remain in the country temporarily while they await their court dates in the US.
Authorities in Tijuana said they would initially be housed in a government-run shelter.
Mexico will not accept unaccompanied minors, people with health problems or those who have appealed a rejected asylum claim, said Velasco.
Trump announced a deal on Friday to reopen the US government after a record five-week shutdown triggered by his fight with Congress over funding for a border wall, which he says is the only way to stop illegal immigration.
The deal with opposition Democrats does not include funding for the wall.
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