Mexico City (AFP): United Nations experts on Friday called on Mexico to crack down on extortion by police and other officials targeting vulnerable migrants in flagrant violation of their human rights.

The UN Working Group on Arbitrary Detention also urged the Mexican government to avoid a repeat of the tragedy last year when 40 migrants died in a fire while behind bars.

More than 240,000 undocumented migrants and asylum seekers were detained in Mexico in the first half of 2023, including many children, the experts said at the conclusion of a 12-day visit.

According to the Mexican constitution, such detentions should be limited to 36 hours, but the UN delegation said that a "significant number" of migrants were held for longer.

The experts were concerned over "frequent reports of bribery and corruption, where individuals who are in a vulnerable position... were asked for money, often by officials, including police officials," group member Matthew Gillett said.

"If they paid, they were allowed to go on their way. If they didn't pay, they would be detained," he told AFP.

"That kind of practice is completely in violation of human rights. It is the kind of practice that the authorities here in Mexico must stamp out," Gillett said.

The working group called on Mexico to ensure that migrants are not held behind locked doors, as was the case at the time of the fire that left 40 migrants dead in Ciudad Juarez near the US border in March.

"One of the concerns there was that the locks on the doors and other chains prevented them from escaping from the building," Gillett said.

"That's an issue that the authorities need to pay close attention to -- to ensure that any individuals that are kept in detention have access to outdoor spaces (and) fire escapes," he said.

When it comes to child migrants, detentions must be "exceptional," Gillett said, voicing concern that tens of thousands of minors were held last year.

"Children should not be placed behind bars unless strictly necessary and for as short a time as possible," he said.

Every year, thousands of US-bound migrants risk their lives making the dangerous journey through Mexico as they flee poverty and violence in their home countries.

Underscoring the risks, two Mexican migrants were killed and three injured Friday when they were shot by unknown assailants while trying to cross to the United States, the national migration agency reported.

Armed people smugglers have previously been reported to operate in the area.

And in southern Mexico, two people died and 27 were injured when a truck carrying more than 50 migrants overturned Thursday in Chiapas state, authorities said.