A US requirement for asylum seekers to apply for an appointment online before crossing the border from Mexico leaves migrants vulnerable to cartel violence, the NGO Human Rights Watch (HRW) said Wednesday.

Under the policy, all asylum seekers -- except unaccompanied minors -- are required to sign up for an appointment with Customs and Border Patrol (CBP) via a mobile phone app or to have had their asylum request denied by another country before trying to cross the US-Mexico border.

Otherwise, any asylum request is presumed illegitimate and migrants risk deportation and a five-year ban on entering the United States.

The rule came into force in May 2023 after the Biden administration repealed a pandemic-era policy known as Title 42, a public health measure used by former president Donald Trump as a blunt tool to block migrants.

"But one material outcome for asylum seekers has remained the same," the HRW report said. "They are forced to wait in northern Mexican states, as well as in many cities in other parts of Mexico through which migrants transit."

There, they are "systematically targeted by cartels, sometimes with the help of Mexican government officials, for kidnapping, extortion, sexual assault, robbery, and other abuse," HRW said.

This online application creates a kind of "digital metering" which "feeds cartel needs for a vulnerable population to prey upon," HRW said in the report, which was based on interviews with 128 asylum seekers, shelter workers, Mexican officials and employees at humanitarian organizations.

The "nearly mandatory" use of this app, called CBP One, means that anyone who presents themselves at the border without having made a digital appointment is turned away by US and Mexican border officials, HRW said.

Many people struggle to use the app, HRW researchers found, often due to technical difficulties or language barriers. The app is only available in English, Spanish or Haitian Creole.

"The Biden asylum rule and digital metering violate the fundamental legal principle of nonrefoulement, which prohibits the return of refugees ... to countries where their lives or freedom would be at risk," the report alleges.

The administration therefore "should immediately rescind its asylum rule, end all practices of metering, digital or otherwise, and stop collaborating with Mexico and other states to block asylum claims in the United States," the report urges.

HRW also recommends that if the US government decides to keep the CBP One app to use as a scheduling tool -- rather than as a metering tool -- it should be updated and include access in more languages, including Arabic, French, Chinese, Russian, Portuguese and Indigenous American languages.