Migrants camped along the US-Mexico border are attempting to use a new online immigration system that has replaced pandemic-era rules, sparking a wave of desperate arrivals.

Authorities say the border has been relatively calm, less than a day after the lifting of the rules known as Title 42, with migrants and officials still assessing the new Biden administration regulations.

Migrants continued to wade into the Rio Grande to take their chances getting into the US while defying officials shouting for them to turn back. Others hunched over phones trying to access an appointment-scheduling app that that is a centrepiece of the new system. Migrants with appointments walked across a bridge hoping for a new life.

The White House says the revamped system is designed to crack down on illegal crossings and to offer a new legal pathway for migrants who often pay thousands of dollars to smugglers to get them to the border.

On Friday, President Joe Biden commended Spanish Prime Minister Pedro Sánchez for his country's collaboration with the US and Canada to establish migration hubs in Latin America where asylum seekers will be able to apply for refuge.

About 100 processing centres are opening in Guatemala, Colombia and elsewhere for migrants to apply to go to the US, Spain or Canada. Up to 1,000 can enter daily through land crossings with Mexico if they snag an appointment on the app.

Migrants are now essentially barred from seeking asylum in the US if they did not first apply online or seek protection in the countries they travelled through.

Biden, who is running for reelection, faces withering criticism from migrant advocates, who say he's abandoning more humanitarian methods, and from Republicans, who claim he's soft on border security.

Title 42 had been in place since March 2020. It allowed US officials to quickly return asylum seekers back over the border on grounds of preventing the spread of COVID-19. While Title 42 prevented many from seeking asylum, it carried no legal consequences, encouraging repeat attempts. Migrants now face being barred from entering the US for five years and possible criminal prosecution.

Border holding facilities were already far beyond capacity in the run-up to Title 42's expiration. Officials had orders to release migrants with a notice to report to an immigration office if overcrowding and other factors became critical.

But late Thursday, a federal judge appointed by former President Donald Trump temporarily halted the administration's plans to release people into the US and set a court date on whether to extend the ruling. Customs and Border Protection said it would comply, but called it a "harmful ruling that will result in unsafe overcrowding."

Other parts of the administration's immigration plan were also in legal peril with a lawsuit by advocacy groups saying the new policy is no different than one adopted by Trump, which was rejected by the same court.

The Biden administration says its rule is different, arguing that it's not an outright ban but imposes a higher burden of proof to get asylum and pairs restrictions with newly opened legal pathways.