The United States bolted tough new immigration policies into place on Friday, setting up an uncertain future for desperate migrants reaching its southern border as a top official expressed confidence the system will hold.

Thousands of people remained in Mexico hoping to enter the United States, as it remained to be seen how new rules for people crossing the border illegally would be enforced.

In Ciudad Juarez, Mexico, some 200 migrants were blocked by US troops from accessing Gate 42, the entry point to El Paso, Texas where hundreds crossed on Thursday.

In Brownsville, Texas, migrants who crossed before Friday were being taken to detention centers for processing, with many hoping to register their names and be released into the country.

Meanwhile US officials reported the death of an unaccompanied migrant child in the custody of Health and Human Services, which takes care of children entering the country unaccompanied.

The department gave no details, but Honduras' Minister of Foreign Affairs Enrique Reina said a 17-year-old Honduran boy had died in an HHS facility in Florida.

"The border is not open," Homeland Security Secretary Alejandro Mayorkas declared at midnight as the United States lifted Title 42, the pandemic-era rule that had allowed officials to summarily expel border crossers since March 2020.

The rule change brings back into effect a decades-old policy known as Title 8, which allows border-crossers to seek asylum – along with strict new measures many fear could expedite deportations and sanctions.

Washington hopes the new rules combined with more legal pathways can bring under control the huge flow of migration to the US southern border, where several hundred thousand have tried to cross every month in the past year.

"People who do not use available lawful pathways to enter the US now face tougher consequences, including a minimum five-year ban on re-entry and potential criminal prosecution," declared Mayorkas.(AFP)