Thousands of migrants have massed at the US-Mexico border in the final hours before pandemic-era immigration restrictions expired, with some rushing to cross ahead of tough new asylum rules.

The COVID-era border rules, known as Title 42, had been tied to the US federal public health emergency that also expired at midnight on May 11 (2pm Friday AEST).

Ahead of the laws lapsing, US Customs and Border Protection said it was holding up to 28,000 migrants at its facilities, far beyond its stated capacity, two federal officials said.

The busiest border detention facilities are in the Rio Grande Valley and El Paso in Texas and two areas in Arizona, according to union President Brandon Judd.

This week, the number of people caught crossing illegally topped 10,000 every day – nearly double the numbers of a fortnight ago.

Due to the high volume of arrivals, agents had been releasing some migrants without a notice to appear in immigration court where they could make an asylum claim, telling them to report to an immigration office later, Mr Judd said.

In Yuma, Arizona, hundreds of migrants lined up in the chilly hours before dawn at a gap in the towering border fence, waiting to turn themselves in to US agents.

Some – like 40-year-old Jovanna Gomez from Colombia – decided to try their luck crossing after hearing about the US policy change.

“In my country, you hear that immigration will only be allowed until May 11, so we came racing against the clock,” she said. “It wasn’t easy.”

Under Title 42, a Trump-era law that had been in place since March 2020, hundreds of thousands of migrants have been quickly expelled to Mexico.

But because Mexico accepted the return of only certain nationalities – mostly its own citizens and Central Americans, and more recently Venezuelans, Cubans, Haitians and Nicaraguans – migrants from other countries were largely allowed in to pursue their immigration claims.

That is set to change as President Joe Biden’s administration sends more personnel and funds to the border while implementing the new regulation.

It denies asylum to almost all migrants who cross illegally. Migrants who are caught crossing illegally are not allowed to return for five years, and can be prosecuted if they try to.

The new laws bar anyone who has passed through another country without seeking refuge elsewhere or who failed to use legal pathways to enter the United States.

Instead, the US has opened up legal options for migrants to apply for entry from abroad, in an effort to discourage people from coming to the border.

It took effect when Title 42 lifted, along with the declared end of the broad COVID public health emergency.

The Biden administration had been blocked by the US Supreme Court from ending the laws earlier, and has been preparing for the change for nearly two years.

Despite that, Mr Biden said this week that the border was “going to be chaotic for a while.”

Border cities have struggled to shelter the new arrivals and provide transportation to other destinations.

“We’re stocking up on food and supplies as much as possible,” said Nicole Reulet, marketing director of Rescue Mission El Paso, a shelter that houses migrants.

“Nobody really knows what to expect, or what the numbers will look like. It makes it hard for us to prepare.”

Far from the border, other cities say they are also struggling to cope. In New York, Mayor Eric Adams temporarily loosened right-to-shelter rules because of strained resources.

Republicans blame Mr Biden for scrapping the restrictive policies of former President Donald Trump, while Biden officials have escalated attacks on Republicans, saying they failed to fix immigration laws or provide adequate border funds.

“I asked the Congress for a lot more money for the Border Patrol,” Mr Biden said on Wednesday. “They didn’t do it.”

A Reuters/Ipsos poll released this week showed that only 26 per cent approved of Mr Biden’s handling of immigration.

In Texas, Republican Governor Greg Abbott, a fierce critic of Mr Biden’s border policies, expanded a National Guard deployment this week “to help intercept and repel large groups of migrants trying to enter Texas illegally”.

-with AAP