Dozens of migrants hoping to cross into the US have died in a horror blaze at a processing centre on the Mexican border.

Mexico’s president said the deadly inferno happened when migrants set their mattresses alight in protest after discovering they would be deported.

At least 39 people were killed — most of them from Guatamala — in one of the most lethal fires to hit Mexico in years.

“They didn’t think that would cause this terrible tragedy,” President Andres Manuel Lopez Obrador told media.

The savage fire happened in the Mexican northern border city of Ciudad Juarez — which is opposite El Paso, Texas — in a facility holding 68 men.

It occurred as the US and Mexico are battling to cope with record levels of border crossings at their shared frontier.

The BBC reports many have been rushing to the border in expectation of an end to a pandemic-era policy which gave the US government the power to quickly expel migrants trying to cross.

A Reuters witness at the scene overnight saw bodies laid out on the ground in body bags behind a yellow security cordon, surrounded by emergency vehicles.

The fire had been extinguished.

“I was here since one in the afternoon waiting for the father of my children, and when 10pm rolled around, smoke started coming out from everywhere,” said 31-year-old Viangly Infante, a Venezuelan citizen.

Her husband, 27-year-old Eduard Caraballo, was in a holding cell inside the facility when the fire started and survived by dousing himself in water and pressing against a door, said Infante, who added she saw many dead bodies lying on the ground.

Recent weeks have featured a build-up of migrants in Mexican border cities as authorities attempt to process asylum requests using a new US government app known as CBP One.

Many migrants feel the process is taking too long and earlier this month hundreds of mostly Venezuelan migrants got into a scuffle with US officials at the border after their frustration welled up about securing asylum appointments.

In January, the administration of US President Joe Biden said it would expand restrictions from his predecessor Donald Trump’s era to rapidly expel Cuban, Nicaraguan and Haitian migrants caught illegally crossing the US-Mexico border in an effort to contain the border flows.

At the same time, the United States said it would allow up to 30,000 people from those three countries plus Venezuela to enter the country by air each month.

White House National Security Council spokeswoman Adrienne Watson on Twitter expressed her sorrow over the “heartbreaking” loss of life and said the US government stood ready to provide Mexico with any necessary support.

The blaze in Ciudad Juarez is one of the deadliest incidents to afflict migrants in Mexico in the past few decades.

In December 2021, at least 55 people were killed and the dozens were injured when a truck packed with migrants flipped over in the southern border state of Chiapas.

Seventy-two migrants were massacred by drug cartel gunmen in the northern Mexican state of Tamaulipas in 2010.

Forty-nine children died following a blaze in a daycare centre in the northern city of Hermosillo in 2009.