US President-elect Joe Biden will roll back Donald Trump's tough approach to undocumented immigrants in sweeping reforms to be introduced on his first day in office, Biden's Homeland Security Department nominee said Tuesday.

Cuban-born Alejandro Mayorkas told a Senate panel that the reforms would include a path to citizenship for millions of longtime US residents without papers, and the possibility of asylum to others escaping abuse and violence.

"President-elect Biden has committed to presenting Congress, on day one, with an immigration reform bill that, once and for all, fixes I think what we all can agree is a broken immigration system," Mayorkas told the Senate Homeland Security Committee.

Biden "has spoken of the need for a path to citizenship for the individuals who have been in this country for many years, who have contributed to our communities and to this nation's economic prosperity," he said.

The Washington Post reported that Biden will unveil broad immigration reforms Wednesday just after he is inaugurated president.

According to the Post, they will include an eight-year track to gain citizenship for immigrants without legal status, and an increase in admissions, which Trump slashed to a trickle.

"I would be privileged to work with Congress to pass immigration reform legislation that provides that path, and provides a permanent solution to what is clearly a broken system," said Mayorkas.

- Asylum -

Mayorkas told the committee, which is vetting his nomination, that if approved as secretary of Homeland Security he would roll back departing Trump's tough measures to seal the southern US border.

Arriving migrants would be permitted to apply for asylum and have their cases reviewed a process the Trump administration virtually ended, with the effect of sharply reducing migrant entry into the country, Mayorkas said.

Under Biden, "There is a commitment to follow our asylum laws, to enforce our asylum laws," he said.

"That means to provide humanitarian relief for those individuals who qualify for it."

But, he added, "it will take time to build the infrastructure and capacity."

Mayorkas could face his first challenge, if approved, with thousands of migrants who set out from Honduras to reach US-Mexico border.

On Monday, security forces broke up the caravan which had amassed in the Guatemalan town of Vada Hondo and on Tuesday thousands were returned to Honduras on buses and trucks, halting their northward march.

Pressed on what he would do if they eventually reach the US border, Mayorkas said the asylum law would be applied.

"This is not the first caravan, that has apparently approached the border over the last four years or ... over the last 12 years," he said.

"When people present themselves at our border we apply the laws of our nation to determine whether they qualify for relief under our humanitarian laws, or whether they don't."

"If they do not qualify to remain in the United States, then they won't."

- Halt to wall building -

Mayorkas, 61, is a former federal prosecutor who ran US immigration services and then was DHS deputy secretary during Barack Obama's administration.

He oversaw the implementation of DACA, the Deferred Action for Childhood Arrivals program, meant to offer a way to citizenship for millions of longtime US residents who entered the country illegally.

If confirmed, Mayorkas said he did not envision radical changes to the sprawling, 240,000-strong DHS bureaucracy, whose mission is focused firstly on protecting the country from terror and cyber attacks and natural disasters.

He rejected calls by some Democrats and human rights groups to eliminate the DHS Immigration and Customs Enforcement agency, which Trump used to round up tens of thousands of people without legal resident papers.

Mayorkas said Biden plans to halt Trump's program to build a wall along the long southern border, but said alternatives to slow illegal immigration include more electronic measures and possibly more personnel in some ares.

But his confirmation could run into some bumps.

After the hearing, Republican Senator Josh Hawley said he would hold up any push for fast confirmation.

"Mr. Mayorkas has not adequately explained how he will enforce federal law and secure the southern border given President-elect Biden's promise to roll back major enforcement and security measures," he said in a statement.

"Given this, I cannot consent to skip the standard vetting process and fast-track this nomination when so many questions remain unanswered."