US President Joe Biden on Wednesday tasked Vice President Kamala Harris with tackling an influx of migrants on the Mexican border, aiming to take charge of a situation that has energized opponents of the new administration.

"I can think of nobody who is better qualified to do this," Biden said at a White House meeting with Harris and other top officials as thousands flee violence in impoverished Central America.

"When she speaks, she speaks for me," Biden said, adding: "I give you a tough job."

Harris said there was "no question that this is a challenging situation."

It marks the first time that Biden has assigned a specific portfolio to Harris, who is seen as representing a younger generation in the Democratic Party and has been constantly at his side since they entered office in January.

A White House team was also joining lawmakers on a tour of a center in Texas for the rising number of unaccompanied migrant children.

Speaking earlier to CBS News, Harris acknowledged that the United States needed to speed up its processing of migrants' asylum claims and care better for them.

"It's a huge problem. I'm not going to pretend it's not," Harris said in the interview.

But she said that the administration, in office for less than 100 days, needed to repair a structure torn apart by former president Donald Trump, who rose to power on an aggressive anti-immigration agenda.

"We have to reconstruct it. It's not going to happen overnight," Harris said of the immigration system.

She also vowed a greater effort to address the "root causes" of the border situation -- helping the so-called Northern Triangle of Guatemala, Honduras and Nicaragua so its people do not feel they need to flee.

The Washington Post, in an analysis looking at data, said there in fact has been no "surge" of migrants entering the United States, with recent numbers in line with seasonal norms before the Covid-19 pandemic.

But more of the migrants are children, officials say, and the border influx has unified Republicans and provided a line of attack against Biden, who in his first weeks has succeeded in his promises to ramp up Covid vaccinations and to push through an economic package that offers relief to most Americans.

Partial shift from Trump

Republicans accuse Biden of encouraging migration through a more welcoming approach than Trump, who in his final year essentially closed the border to all undocumented migrants by citing a public health emergency from Covid-19.

Top House Republican Kevin McCarthy has spoken of a "Biden border crisis" and former secretary of state Mike Pompeo tweeted that Biden "supports restrictions on pretty much everything except the border."

Biden halted construction on Trump's cherished wall on the Mexican border and has moved to wind down a controversial Trump policy, which migrant advocates say violates international conventions, in which asylum seekers are forced to wait in Mexico while their cases are heard.

Biden has also faced concern from fellow Democrats who are alarmed at the conditions for children.

In one of Trump's most draconian actions which his aides saw as a deterrent, the United States separated thousands of Central American children from their parents.

While the Biden administration is still expelling adults who arrive without documentation, it has refused to send back into harm's way children who are unaccompanied by a guardian.

Secretary of Homeland Security Alejandro Mayorkas said the administration was setting up more temporary shelters for children, acknowledging the United States was falling short in its goal of transferring children out of custody of border guards within 72 hours.

Critics accuse Biden of replacing Trump's unapologetically harsh measures with an approach of hiding the treatment of migrants by restricting media access.

The White House said it was committed to transparency and would work to let in more journalists on the ground.

It said one television network would pool coverage of Wednesday's tour by White House officials and lawmakers of the center in Carrizo Springs, Texas.