US Homeland Security Secretary Alejandro Mayorkas on Wednesday defended the Biden administration's handling of immigration, amid a surge at the Mexico border that is expected to grow when pandemic limits are dropped.
"We inherited a broken and dismantled system that is already under strain," Mr Mayorkas told the House appropriations homeland security subcommittee.
"It is not built to manage the current levels and types of migratory flows."
He urged Congress to pass long-term reforms.
His testimony came as the administration locked horns with a federal court in Louisiana that granted a request by Republican-controlled states to temporarily halt their planning to wind down Title 42.
The order prevents the government from taking any action before a hearing on May 13 to determine whether Title 42 can be lifted.
Mr Mayorkas set out a blueprint to boost border personnel, strike at people smugglers, speed up processing and increase holding capacity, as he warned the worst was yet to come.
"A significant increase in migrant encounters will strain our system even further and we will address this challenge successfully," he said.
Enacted at the start of the pandemic in March 2020 under US president Donald Trump, Title 42 requires border agents to turn away any undocumented migrants who cross into the US, leading to the expulsion of more than 1.7 million.
The decision on dropping Title 42 was made by the US Centres for Disease Control and Prevention, which said it was "no longer necessary" because of "an increased availability of tools to fight Covid-19".
But US President Joe Biden and Mr Mayorkas are taking the heat for the growing controversy.
Conservatives complain that the Mayorkas plan does not include any major new expenditure, any prospect of the CDC reversing its Title 42 decision, or a pledge to complete Mr Trump's controversial border wall.
Meanwhile border-state Democrats up for re-election in the November midterms have voiced frustration about the White House's handling of the issue, accusing it of failing to come up with a Plan B.
The Senate was deadlocked on Wednesday over a push to pass legislation that would keep Title 42 from ending.
Republicans are likely to demand a vote to reinstate the measure as part of any action to approve the next tranche of Covid-19 relief, and both of those initiatives could end up entangled in legislation to approve further aid to Ukraine.
"It is inappropriate to use a public health law — Title 42, that deals principally with quarantines — to try to deal with an immigration challenge that we face," Health and Human Services Secretary Xavier Becerra told the House energy and commerce health subcommittee.
"It's like using a knife to do work that you would use a screwdriver to do."
- AFP contributed to this report