Democrat Biden pledged during his White House race with Trump in 2020 that he would abandon the Republican's signature policy and would not build any more of the wall.

But his own Department of Homeland Security on Thursday announced the building of a new section in southern Texas to tackle an "acute and immediate need," as migrant crossings surge.

Biden, who is polling neck-and-neck with rival Trump ahead of a likely 2024 election rematch, insisted his predecessor had tied his hands on the wall-building.

"They have to use the money for what it was appropriated for. I can't stop that," he told reporters in the Oval Office.

He said the cash was earmarked for the border wall by the US Congress under Trump in 2019, and that lawmakers had since rejected his appeals to reassign the funds.

Asked if he thought the border wall was effective, Biden replied simply: "No."

'No choice' 

The new section of wall will be built in the "high illegal entry" Rio Grande Valley sector of the US-Mexico border, where there have been more than 245,000 attempted illegal entries this fiscal year.

But Homeland Security Secretary Alejandro Mayorkas later said the announcement had been "taken out of context."

"The action that we took -- we had no choice," Mayorkas said during a news conference in Mexico with Secretary of State Antony Blinken. 

"There is no new administration policy with respect to the border wall." 

Illegal immigration has become a major political headache for Biden, with opposition Republicans accusing him of lax border policies.

The border issue has even added to uncertainty over US aid for Ukraine, with some Republicans refusing to approve funds until Biden acts on migrants.

Meanwhile some Democrats including progressive lawmaker Alexandria Ocasio-Cortez criticized the wall decision. So did conservationists after it emerged that two dozen federal environmental laws would be waived to build the extension.

The White House said it was "absolutely false" that there had been a reversal by the president. 

"We have to comply by law and that's what we're doing," Press Secretary Karine Jean-Pierre told reporters.

'I was right' 

Trump, the frontrunner for the 2024 Republican presidential nomination, said the Biden administration move showed "I was right when I built 560 miles... of brand new, beautiful border wall."

"Will Joe Biden appolgize (sic) to me and America for taking so long to get moving, and allowing our country to be flooded with 15 million illegals (sic) immigrants, from places unknown," Trump said on his Truth Social platform.

Mexican President Andres Manuel Lopez Obrador criticized the US plan, calling it a "setback" ahead of talks with Blinken.

Their talks came after a spike in the number of people -- mostly from Central America and Venezuela -- seeking to cross from Mexico into the United States.

The Biden administration announced later Thursday that it would resume direct deportation flights to Venezuela, which is still under US sanctions for rights abuses.

"This comes following a decision from the authorities of Venezuela to accept back their nationals," said a senior administration official.