The United Nations said it was "profoundly concerned" by the new asylum restrictions on the US-Mexico border unveiled Tuesday by US President Joe Biden and urged Washington to think again.

Biden ordered sweeping new migrant curbs on the southern US border in a bid to neutralise one of his political weak spots in his re-election battle against Donald Trump.

The executive order bans migrants who enter the country illegally from claiming asylum when numbers surge past 2,500 in a day, as well as making it easier to deport them back to Mexico.

"UNHCR, the UN Refugee Agency, is profoundly concerned about the new asylum measures announced today by the United States which impose harsh restrictions on the right to seek asylum in the country," it said in a statement.

"The new measures will deny access to asylum for many individuals who are in need of international protection, and who may now find themselves without a viable option for seeking safety and even at risk of refoulement," being sent back across the border, it added.

"Any person who claims to have a well-founded fear of being persecuted in their country of origin must have access to safe territory and have this claim assessed before being subject to deportation or removal.

"We call on the United States to uphold its international obligations and urge the government to reconsider restrictions that undermine the fundamental right to seek asylum."

Migrants entering the United States are normally allowed to claim asylum if they face harm or persecution on the grounds of race, religion, nationality, political opinion, or membership in a particular social group.

But many spend years waiting for their claims to be processed, with critics saying that people often cross purely for economic reasons and then game the system to remain in the United States.

The UNHCR said it recognised that the United States was facing challenges in dealing with the significant number of people arriving at its border.

"The United States has a long history of welcoming refugees and continues to do so through various pathways, including resettlement, private sponsorship, and others," it said.

"We remain committed to supporting the United States in much-needed broader reform efforts, including to improve the fairness, quality, and efficiency of its border management and asylum systems."