The United States will temporarily close its Mexico border to asylum seekers from Wednesday, as President Joe Biden tries to neutralize his political weakness on migration ahead of November's election battle with Donald Trump.

The 81-year-old Democrat signed a long-awaited executive order taking effect at midnight to "gain control" of the southern frontier with Mexico, after record numbers of illegal border crossings sparked concerns among voters.

"I've come here today to do what the Republicans in Congress refuse to do -- take the necessary steps to secure our border," Biden said in a brief address at the White House, flanked by officials from border states.

Biden's executive order bars migrants who enter the US illegally from claiming asylum when numbers surge past 2,500 in a day -- a threshold that has already been passed. It also makes it easier to deport people back to Mexico.

The curbs will remain in place until numbers fall back down below 1,500 illegal crossings a day.

Biden came under fire from all sides for the policy shift, which uses the same law that Trump once employed to ban migrants from Muslim countries when he was president.

US Republicans immediately slammed the move as too little -- while rights groups said they would go to court to stop the most drastic migration policy of any Democratic president for decades.

The UN refugee agency said it was "profoundly concerned" by Biden's measures.

Trump -- whose signature policies included a border wall that also failed to tackle the problem -- accused his rival of having "surrendered" the border to illegal immigration.

"Millions of people have poured into our country -- and now, after nearly four years of his failed, weak leadership, pathetic leadership, crooked Joe Biden is pretending to finally do something about the border," Trump, 77, said in a video posted to his Truth Social media platform.

He added the border restrictions were all for "show" ahead of their presidential debate later this month.

Republican House Speaker Mike Johnson, a Trump ally, quickly labeled the move as "window dressing."

'Extremely cynical'

Under Biden, illegal crossings of the 1,900-mile border have surged to record levels, rising to a monthly peak of around 300,000 -- 10,000 a day -- in December.

Most are from Central America and Venezuela as they flee poverty, violence and disasters exacerbated by climate change, but growing numbers are also coming from other parts of the world to Latin America before making the treacherous trip north to the United States.

The numbers have fallen considerably in recent months, to some 179,000 in April, but polls show it is still one of Biden's biggest electoral liabilities.

Biden slammed Trump and Republicans for "weaponizing" migration by blocking his request for billions of dollars in border funding in what he called an "extremely cynical, political move."

He also addressed his critics on the left, insisting that he would not "demonize" migrants and adding: "For those who say the steps I've taken are too strict, I say to you... be patient."

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 game the system to remain in the United States.

A senior White House official moved to defuse criticisms that Biden is effectively copying Trump, saying that while in office, the Republican "demonized immigrants, instituted mass raids, separated families at the border and put kids in cages. Their policies went against our values as a nation."

Trump has drastically ramped up his anti-immigration rhetoric as he seeks a White House comeback.

Biden spoke to his outgoing Mexican counterpart Andres Manuel Lopez Obrador on Tuesday and thanked him for helping "manage migration at our shared border," the White House said.

The US president spoke to president-elect Claudia Sheinbaum, Mexico's first woman leader, on Monday.