President Joe Biden on Thursday warned undocumented migrants to stay away from the US border while opening the door to limited legal arrivals from four impoverished countries, in a delicate balancing act on one of the country's most explosive political issues.

Biden, who is expected soon to declare his bid for a second term, is seeking to thread the needle on an issue where he faces pressure both from anti-immigration Republicans and from leftwing Democrats arguing for greater human rights.

Calling the US immigration system "broken," Biden also announced he will visit the US-Mexico border town of El Paso in Texas on Sunday, on the eve of a North American leaders' summit in Mexico City where border security will be a key topic.

Under his new plan, Biden said a controversial rule called Title 42 would be expanded to allow border guards to immediately turn away more would-be migrants if they arrive by land.

"Do not just show up at the border," Biden warned in a White House speech.

The toughened measures will stem record numbers of migrant and asylum seekers arriving after epic, often dangerous journeys organized by human smugglers, the White House hopes.

To try and placate critics on the left, the president said up to 30,000 qualifying migrants will be allowed into the United States each month from Cuba, Haiti, Nicaragua and Venezuela.

They must apply in their home countries, have a US sponsor, and undergo background checks.

"It's safe and it's humane, and it works," Biden said.

However, he said fully fixing the system requires Congress to legislate and increase funding for the border and immigration infrastructure.

And he hit out at congressional Republicans for refusing to consider more comprehensive reforms put forward by his administration, blaming "extreme Republicans" for the years-long impasse, and for using immigration problems as an election issue.

- Title 42 controversy -

The White House proposals unveiled Thursday expand on a similar program of limited legal access for Venezuelans which began in October. However, Title 42 use is at the center of a fierce debate.

The measure was implemented under Donald Trump's administration, ostensibly as a health measure to reduce the flow of people during the Covid pandemic. However, human rights advocates and many in Biden's Democratic party said the use of Title 42 was an abuse of a health emergency by an openly anti-immigration president.

Biden's administration is actually seeking to remove the rule and is now waiting for the Supreme Court to decide whether it should stay or go. So the new embrace of the tool for restricting migrants -- it allows border guards to send back illegal crossers without any further process -- immediately caused uproar.

"President Biden correctly recognized today that seeking asylum is a legal right and spoke sympathetically about people fleeing persecution. But the plan he announced further ties his administration to the poisonous anti-immigrant policies of the Trump era," said Jonathan Blazer of the American Civil Liberties Union.

Wendy Young, head of Kids in Need of Defense, which advocates for unaccompanied refugee and migrant children, said the organization was "deeply disappointed that the Biden Administration is once again limiting desperate asylum seekers' access to humanitarian protection at the border."

- No easy fix -

The US economy relies heavily on immigrant labor. However, uncontrolled migration over the long Mexico border, including large numbers of asylum seekers and also illegal crossers traveling across often dangerous terrain, has strained the system beyond easy repair.

Multiple presidents have failed to resolve the exceptionally complex issue.

Trump rose to power in 2016 in large part on his message that criminals, including rapists and murderers, were pouring across the border. The incendiary rhetoric struck a chord in communities already nervous about crime or loss of jobs.

Biden, who defeated Trump in 2020, took office vowing to restore traditional US values at the border, giving refuge to asylum seekers and ending harsh detention policies for illegal border crossers.The issue is one of Biden's main political weaknesses as he prepares for what aides say will be his likely announcement of a bid for a second term in office.