The Biden administration is scrambling to manage a growing humanitarian and political challenge at the US-Mexico border that threatens to overshadow its ambitious legislative agenda.

With the number of migrants surging, administration officials say Biden inherited an untenable situation that resulted from what they say was President Donald Trump's undermining and weakening of the immigration system.

But as Congress pivots to immigration legislation, stories of unaccompanied minors and families trying to cross the border and seeking asylum and of overwhelmed border facilities have begun to dominate the headlines, distracting from the White House's efforts to promote the recently passed $1.9 trillion Covid-19 relief bill.

Biden told reporters Sunday at the White House that "at some point" he would go to the border and that he knows what is going on in the border facilities.

"A lot more, we are in the process of doing it now, including making sure we re-establish what existed before, which was they can stay in place and make their case from their home countries," Biden said upon returning from a weekend at Camp David.

The White House dispatched Homeland Security Secretary Alejandro Mayorkas to four Sunday news shows in an effort to stress that it was working to get things under control.

"Our message has been straightforward — the border is closed," Mayorkas said. "We are expelling families. We are expelling single adults. And we've made a decision that we will not expel young, vulnerable children."

The White House has steadfastly refused to call the situation a "crisis", leading to a Washington battle over the appropriate description of the tense situation.

In the first days of his term, Biden acted to undo some of Trump's measures, a rollback interpreted by some as a signal to travel to the United States. While the new administration was working on immigration legislation to address long-term problems, it didn't have an on-the-ground plan to manage a surge of migrants.

"We have seen large numbers of migration in the past. We know how to address it. We have a plan. We are executing our plan and we will succeed,” Mayorkas said.

But, he added, "it takes time" and is "especially challenging and difficult now" because of the Trump administration's moves. "So we are rebuilding the system as we address the needs of vulnerable children who arrived at our borders."

Biden officials have done away with the "kids in cages" imagery that defined the Trump family separation policy — though Trump used facilities built during the Obama administration — but have struggled with creating the needed capacity to deal with the surge.

Officials are trying to build up capacity to care for some 14,000 migrants now in federal custody — and more likely on the way. Critics say the administration should have been better prepared.

"I haven't seen a plan," said Rep. Michael McCaul, R-Texas. "They have created a humanitarian crisis down here at this border that you have seen now. And the reason why they are coming is because he says words do matter, and they do. The messaging is that if you want to come, you can stay."

The administration also has been pressed as to why it will not allow media to see the facilities at the border.

Mayorkas said the government was "working on providing access so that individuals will be able to see what the conditions in a Border Patrol station are like."

But Sen. Tom Cotton, R-Ark., told "Fox News Sunday" that "it's rich that Secretary Mayorkas won't let press travel with him to the border, but he will come on your Sunday morning show and peddle the same kind of nonsense that has created the Biden border crisis in the first place."

Since Biden's inauguration on January 20, the US has seen a dramatic spike in the number of people encountered by border officials.

There were 18,945 family members and 9,297 unaccompanied children encountered in February — an increase of 168 per cent and 63 per cent, respectively, from the month before, according to the Pew Research Center. That creates an enormous logistical challenge because children, in particular, require higher standards of care and coordination across agencies.

Among the reasons for the surge: thousands of Central American migrants already stuck at the border for months and the persistent scourge of gang violence afflicting Northern Triangle countries — Guatemala, Honduras and El Salvador.

Still, the encounters of both unaccompanied minors and families are lower than they were at various points during the Trump administration, including in spring 2019.

Pointing to the urgency of the situation at the border, Sen. Dick Durbin, D-Ill., expressed confidence that enough Republicans would vote to pass an immigration overhaul.

"We go into this debate, whether it's a crisis or a challenge at the border. Let me tell you, the crisis. We need to address our immigration laws in this country that are broken," said Durbin. "What we see at the border is one exhibit of it, one exhibit of evidence in that. But there's more across the board."

The White House also points to Biden's decision to deploy the Federal Emergency Management Agency to support efforts to process the growing number of unaccompanied migrant children arriving at the border.

Mayorkas appeared on Fox, ABC's "This Week", CNN's "State of the Union" and NBC's "Meet the Press", while McCaul was on ABC and Durbin on CNN.