The US border with Mexico will be "chaotic for a while," President Joe Biden acknowledged Tuesday, days before pandemic-era rules that have made claiming asylum at the frontier all but impossible are due to end.

Tens of thousands of people are massing at border crossing points awaiting the expiration of Title 42 overnight Thursday into Friday.

Thousands have already entered the United States, squeezing through the holes that pockmark the division between the world's wealthiest country and its poorer neighbor, and a spike in asylum claims is expected.

The Texan cities of El Paso, Brownsville and Laredo have declared a state of emergency as they struggle to cope with hundreds of people -- most from Latin America, some from China, Russia and Turkey -- who are already there.

Asked if the United States was ready for a surge in people, Biden told reporters: "It remains to be seen. It's going to be chaotic for a while."

Title 42 was put in place under Donald Trump, ostensibly to prevent people with Covid-19 from entering the country.

In practice, it has been little more than a crude tool to swiftly expel those trying to migrate.