US President Joe Biden has put Vice-President Kamala Harris in charge of controlling migration at the southern border following a big influx of new arrivals.

Mr Biden said he was giving her a "tough job" but that she was "the most qualified person to do it".

The numbers of people arriving have grown since Mr Biden took office.

They include hundreds of unaccompanied minors who are being held in immigration detention facilities.

Mr Biden's predecessor, Donald Trump, was widely criticised over his government's treatment of young migrants at the US-Mexico border.

Since January, the Biden administration has reversed a policy of turning away unaccompanied children, instead choosing to process them and place them with sponsoring families in the US.

But Mr Biden's critics say his policies have led to a surge in illegal migration.

In February, US Customs officials took more than 100,000 people into custody along the southern border, a 28% increase on the previous month.

Announcing Ms Harris's appointment as his immigration czar, Mr Biden told reporters and officials at the White House: "She's the most qualified person to do it, to lead our efforts with Mexico and the Northern Triangle [Honduras, Guatemala and El Salvador], and the countries that are going to need help in stemming the movement of so many folks - stemming the migration to our southern border".

Mr Biden said Ms Harris's past work as California's attorney general made her well suited to leading the effort, adding: "When she speaks, she speaks for me."

In response Ms Harris said: "Needless to say, the work will not be easy. But it is important work."

Many of those arriving at the border have fled poverty and violence in Central America.

In an interview with CBS on Wednesday, Ms Harris said there was a need "to deal with the root causes... of what's happening in the Northern Triangle".

"Dealing with what we need to do around aid in a way that is about developing those countries so that we also deal with the cause of why people are coming into our country," she said.
Analysis box by Anthony Zurcher, North America reporter
A portfolio laced with political opportunity - and peril

For the first two months of the Biden administration, Vice-President Kamala Harris had the look of a presidential understudy, accompanying her boss to major events but never stepping into the spotlight on her own.

That just changed.

By putting her in charge of addressing the growing humanitarian crisis on the US-Mexico border, Biden has given Harris a significant portfolio laced with both political opportunity and peril. Her challenge is to prove that there is way to stem the tide of undocumented migrants coming to the US from Central America without resorting to what Democrats characterise as the Trump administration's draconian policies.

If she succeeds, Harris defuses an issue that the Republican Party - and Donald Trump, in particular - has used as an effective political weapon against her party, earning the gratitude of influential immigration activists. If she fails, the ensuing political fallout could derail the Biden presidency and overshadow all its early accomplishments.

Biden boasted that Harris, as a former California attorney general, is the person "most qualified" to handle the complex political, logistical and diplomatic challenges this immigration issue presents.

Harris now has the chance to prove it - and burnish any future presidential credentials along the way.

Earlier this week, pictures that emerged from a detention centre in Texas showed children huddled together in crowded makeshift rooms. They were sleeping on thin mattresses on the floor under foil blankets.

The government-run tent city in Donna at the US-Mexico border, is reportedly housing 1,000 people.

The photos, said to have been taken last weekend, also raised concerns over how much social distancing was taking place at the centre.

Journalists have not been allowed inside the detention centres since Mr Biden took office, although the White House has said that they will be.

On Wednesday, White House press secretary Jen Psaki said the administration was "committed to transparency and we'll continue to work with agencies on creating avenues for media access to and visibility into these facilities".

Since taking office, Mr Biden has ended construction work on Mr Trump's border wall and called for reviews of legal immigration programmes terminated by the former president.