US President Joe Biden urged migrants not to come to the United States, as criticism mounted over a surge in people arriving at the southern border with Mexico -- including thousands of unaccompanied children.

"Yes I can say quite clearly don't come over ... Don't leave your town or city or community," he said in an interview with ABC News, addressing the migrants.

Speaking hours after his head of homeland security defended the administration's immigration policies on Tuesday, Biden also shrugged off claims that his dismantling of former president Donald Trump's tough stance had encouraged the surge, pointing out that there had been similar surges in 2019 and 2020.

"The idea that Joe Biden said 'come' - I heard the other day that they're coming because I'm a nice guy... Here's the deal, they're not," he said.

On January 20, his first day in office, Biden scrapped several of Donald Trump's contentious immigration policies, including halting new construction of a border wall and proposing legislation to create a citizenship pathway for the nearly 11 million people living illegally in the US.

Republican critics say Biden's policies caused a sharp increase in migrants seeking to cross into the US illegally. 

The president spoke a day after top Republican congressman Kevin McCarthy visited the border in Texas with fellow Republican lawmakers and accused Biden of creating a "crisis."

- Migrant spikes 'not new' -
Biden's chief of homeland security Alejandro Mayorkas on Tuesday also defended the administration's immigration policies.

Mayorkas acknowledged the United States was "on pace" to encounter more migrants at the border than at any time in two decades, but said such spikes were "not new," having also occurred in 2019, 2014 and earlier.

"The situation we are currently facing at the southwest border is a difficult one," Mayorkas said in a statement. 

"We are keeping our borders secure, enforcing our laws, and staying true to our values and principles."

Mayorkas said the rise in unaccompanied children -- some as young as six or seven -- comes from ending the policies of Trump, whose administration "cruelly expelled young children into the hands of traffickers."

"They are vulnerable children and we have ended the prior administration's practice of expelling them," he said.

The Biden administration continues to expel most single adults and people traveling in families.

In February, the US Customs and Border Protection agency (CBP) arrested about 100,000 people at the southern border -- including nearly 9,500 unaccompanied children -- a 28 percent jump over January.

Mayorkas blasted the Trump administration for having cut aid to El Salvador, Guatemala and Honduras meant to tackle the root causes of migration such as violence and the impacts of natural disasters.

Holding facilities for apprehended migrants are crowded, Mayorkas said, noting authorities had not "had the capacity to intake the number of unaccompanied children we have been encountering."

Complicating conditions, pandemic-related physical distancing protocols have further reduced space, said Mayorkas, who has asked the Federal Emergency Management Agency (FEMA) to help activate additional facilities.

Cuban-born Mayorkas said his parents, who brought him to the US as an infant, "understood the hope and promise of America."

"Today, young children are arriving at our border with that same hope," he said. "We can do this."