As the United States begins imposing border rules making it more difficult for migrants to claim asylum, many will most likely face swift deportation to Mexico, where they will be vulnerable to criminal groups and corrupt officials, according to human rights groups.

Mexico’s role as Washington’s enforcement arm to deter migrants from heading illegally to the United States through Mexican territory will become more significant with the lifting on Thursday of a Covid-era policy known as Title 42, which halted the entry of many migrants at the border and allowed the U.S. authorities to rapidly expel them.

In talks last week with the Biden administration, Mexico said it would accept non-Mexican migrants sent back from the United States under the new rules and would process them for Mexican asylum.

But if the asylum system in the United States is plagued by backlogs, the situation in Mexico is just as bad, with asylum cases lingering for years without resolution.

And many migrants expelled to Mexican cities along the U.S. border face daily horrors at the hands of criminal organizations and, in some cases, the same government agencies that Washington is leaning on to help stanch the flow of migrants at the border, according to human rights groups.

Since President Biden took office in January 2021, there have been nearly 13,500 attacks against people deported to Mexico from the United States or blocked from crossing the border, according to a recent report from Human Rights First, an advocacy group.

The report said that, in some cases, Mexican officials have colluded with criminal organizations to extort migrants.

Mexico’s National Migration Institute and the Foreign Ministry did not respond to requests for comment about the government’s treatment of migrants.

“This country is not a safe country,” Yuri Hurtado, a 26-year-old Colombian migrant, said of Mexico.

She left her country in March with six members of her family to escape poverty and violence. She spends her days at a migrant shelter near the U.S. border listening to threatening phone messages from members of a criminal group who, Ms. Hurtado said, kidnapped her relatives last week while they were riding a bus through Mexico.

The shelter where Ms. Hurtado is staying, Casa Migrante San Juan Diego, is in Matamoros, a northern Mexican city that is notorious for violence and across the border from Brownsville, Texas.

Ms. Hurtado said the criminal group holding her two sisters, a brother-in-law and two nephews, who are 2 and 5, had demanded she pay $4,000 for their release or it would start harvesting their organs.

The sum is more than Ms. Hurtado said she could ever afford. The local police, she said, did not help her when she tried to file a report, a typical response by the authorities, according to migrant rights groups.

“It gives me so much fear what happens on the border and, yet, also I am full of fear that I will die alone on the border,” she said, adding that she hoped her relatives would be released before she tried to cross the border.

Stories like Ms. Hurtado’s are not unusual; criminal groups often impose fees on migrants to travel through Mexico and then kidnap them. More than 2,000 migrants were kidnapped by criminal organizations last year, the Mexican government said last week.

At the same time, migrants are also vulnerable to being victimized by Mexico’s migration authorities.

“The abuses by state officials themselves is systemic,’’ said Julia Neusner, a lawyer who co-wrote the Human Rights First report. “We heard hundreds and hundreds of stories from people who experience harm directly at the hands of these state officers, including kidnappings, rape, sexual assault, robbery, extortion.”

When President Andrés Manuel López Obrador took office at the end of 2018, he vowed that Mexico would never be used as a cudgel to “do the dirty work” of Washington’s migration policy.

Instead, his government issued more visas to allow migrants to travel freely to Mexico and make their way to the U.S. border.

But Mr. López Obrador soon discovered, like other Mexican presidents before him, that it is nearly impossible for Mexico to forge a migration policy on its own.

By June 2019, President Donald J. Trump was threatening to slap tariffs on Mexico unless Mr. López Obrador clamped down on the thousands of migrants using Mexican humanitarian visas to head to the United States.

Mr. López Obrador acted swiftly, deploying thousands of troops to Mexico’s northern and southern borders to prevent migrants from entering the country or traveling easily to the United States. The Mexican National Guard, a militarized police force, was given the authority to detain migrants, a power that had been largely concentrated in the hands of migration officials.

“The U.S. migration policy has mobilized the Mexican government for enforcement,’’ Ms. Neusner said. “It is exporting our own border enforcement.”

The closing of legal routes within Mexico and pathways to the United States forced more migrants into the hands of ruthless smugglers, rights groups said.

Mexico’s closer alignment with the United States on enforcement has also led to a shift in the government’ attitude toward migrants, some analysts said.

“The priority is no longer that of human rights and development and protection, as we started out, but due to pressure from the United States, containment, detentions and expulsions were prioritized,” said Tonatiuh Guillén, who was the first commissioner of Mexico’s National Migration Institute under Mr. López Obrador until he was replaced by the former head of Mexico’s federal prison system.

“Deploying the armed forces as your main migration enforcement tool sends a message both to migrants, asylum seekers and to society that migrants are a threat and they should be treated as a security issue, like an invasion,” said Stephanie Brewer, the Mexico director at the Washington Office on Latin America, a research institute.

“That undermines and weakens protections for their physical safety,” she added.

At the Casa Migrante San Juan Diego shelter in Matamoros, half a dozen migrants said this week that either they or a family member had been kidnapped in recent days. They were afraid to venture out of the shelter after dark, fearing the criminal groups that stalk the streets.

The shelter’s director, Jose Luis Elias Rodriguez, said he and his employees had themselves been threatened by criminal groups.

But he vowed to keep helping migrants.

“If we leave, who helps immigrants?” he asked. “Who lends a hand if we leave? Who raises it if we leave? Who stands up for them if we leave?”

Geysha Espriella and Meridith Kohut contributed reporting from Matamoros, Mexico.