El Paso (United States) (AFP) - Pandemic-era rules that have prevented migrants from claiming asylum at the United States’ southern border will expire Thursday, with tens of thousands of people expected to make their case over the coming weeks, further inflaming America’s already heated immigration debate.

Troops and National Guardsmen have been sent to help border control officers handle an expected surge of mainly poor people seeking refuge in the world’s wealthiest country.

For more than three years the 2,000-mile (3,200-kilometer) frontier with Mexico has been all but closed under Title 42 – a health provision designed to keep infection at bay that allowed border guards to turn people away before they made a claim for asylum.

But with the formal ending of the Covid emergency, normal service is set to resume at the stroke of midnight in the US capital.

“But we don’t know what’s coming in the next day, we don’t know what’s coming in the next 10 days,” said Oscar Leeser, the mayor of the Texas city of El Paso, which is routinely one of the busiest crossings on the border.

“We know that they’ll continue to come and we’ll continue to make sure that we help them.”

Republican Party opponents have hammered President Joe Biden for allowing Title 42 to lapse, claiming the Democrat is throwing open the country’s doors to migrants.

The party’s standard bearer, former president Donald Trump said it would be a “day of infamy”.

“You’re going to have millions of people pouring into our country,” he said, suggesting that if he retakes the White House next year he would reinstitute a policy of separating families at the border.

“When you have that policy, people don’t come,” he told a CNN townhall on Wednesday.

Wary of the political cost of the endlessly looped footage of migrants climbing through holes in the border, the Biden administration has ramped up the number of border security personnel in the area.

About 24,000 border police and 1,100 processing staff have been activated, while the Pentagon has committed around 4,000 troops.

But with pressure from their own side for a more humane border policy, the White House has sought to balance sticks with carrots.

“Our overall approach is to build lawful pathways for people to come to the United States, and to impose tougher consequences on those who choose not to use those pathways,” Homeland Security Secretary Alejandro Mayorkas said.

Rules in force from Friday will mean anyone who enters the country illegally will face a much tougher task to prove their case for asylum, and will in many cases be deported, Mayorkas said.

That threat appeared to be percolating through to some migrants in Matamoros, over the border from Brownsville, Texas.

Venezuelan Andres Sanchez told AFP he would not be chancing an illegal river crossing.

“We will lose all rights to legal process. They can automatically throw us back because we entered illegally,” he said.

But even for those obeying the rules, the glitchy government app they are required to use to start their asylum claim makes life even harder.

“Look, it’s stuck,” said Ronald Huerta, a Venezuelan in the Mexican city of Ciudad Juarez who on Wednesday could not get past the application’s language settings.

For Jeremy De Pablos and others with dark complexions, the app seemed to have trouble logging their faces.

“It’s like a game of chance,” he said, sighing. “It recognizes who it wants to.

“It’s amazing that an app practically decides our lives and our future.”