Under the order, which took effect at midnight, officials can quickly remove migrants entering the US illegally without processing their asylum requests.

That will happen once a daily threshold is met and the border is "overwhelmed", the White House said in a statement.

Rival Republicans say Mr Biden has not gone far enough, while some of the president's Democratic allies - and the United Nations - have expressed concern.

A spokeswoman for the UN's refugee agency said those fearing persecution should have access to safe territory.

Mr Biden spoke about the order at an event on Tuesday with several border town mayors. He said "this action will help us gain control of our border".

The president criticised Republicans for not passing bipartisan immigration reform in Congress earlier this year - and asked left-wing critics of the new executive action to "be patient".

"We're wearing thin right now," he said. "Doing nothing is not an option."

The order - which also aims to speed up cases and ease pressure on overburdened US immigration courts - has met criticism from activists.

"It’s unfortunate that politics are driving the immigration conversation in an increasingly restrictive direction," said Jennie Murray, president and the CEO of the National Immigration Forum.

More than 6.4 million migrants have been stopped crossing into the US illegally during Joe Biden's administration.

The arrival numbers have plummeted this year, though experts believe that trend is unlikely to continue.

Mexican media have depicted the move as one of Mr Biden's toughest policies, though President Andrés Manuel López Obrador sought to downplay the issue, arguing that economic and cultural exchange made a border closure "impossible".

Authorities in Tijuana asked what would happen to asylum seekers denied entry to the US.

Shelters in the Mexican city could quickly get overcrowded, one local official warned. "We’d start seeing people on the streets, sleeping in tents”.

About a dozen advocates and Democratic lawmakers had their own press conference outside the US Capitol on Tuesday, criticising Mr Biden's decision.

Pramila Jayapal, the chair of the Congressional Progressive Caucus, said she was "profoundly disappointed" by the executive action and called it a "step in the wrong direction".

Others in Mr Biden's party disagreed - including Ruben Gallego, an Arizona congressman, who said there was still "more work to do".

The campaign for Donald Trump, Mr Biden's Republican challenger for the presidency, earlier argued that the order “is for amnesty, not border security”.

In Tuesday's statement after the president's announcement, the White House said that the new order "will be in effect when high levels of encounters at the southern border exceed our ability to deliver timely consequences, as is the case today."

Among the actions announced on Tuesday were the use of a 1952 law that allows access to the American asylum system to be restricted.

The law, known as 212(f), allows a US president to "suspend the entry" of foreigners if their arrival is "detrimental to the interests" of the country.

The same regulation was used by the Trump administration to ban immigration and travel from several predominantly Muslim countries and to bar migrants from asylum if they were apprehended crossing into the US illegally, provoking accusations of racism.

"While there’s no question the US needs to better address challenges at the border, the use of 212(f) authority is concerning," Ms Murray said.