WASHINGTON -- U.S. President Joe Biden's head of Homeland Security sparred Wednesday with members of Congress over the surge of migrants at the Southwest border, refusing to concede the situation was a crisis or even much different from what the two previous administrations faced.

Secretary Alejandro Mayorkas gave ground on two Republican points as he acknowledged the administration may not have adequately notified communities chosen to host facilities for migrant teens and children and said some people were released without being tested for COVID-19, though a new testing policy has been implemented.

But Mayorkas, who remained largely unflappable during nearly four hours of often hostile interrogation, repeatedly deflected Republicans who sought to cast the situation along the U.S.-Mexico border as out of control.

"We have a very serious challenge, and I don't think the difficulty of that challenge can be overstated," Mayorkas said. "We also have a plan to address it. We are executing on our plan and we will succeed."

It was the first high-profile immigration showdown for the new administration, which is facing political blowback as it copes with the sharp increase in migrants at the same time it attempts to undo some of President Donald Trump's signature actions to reduce both legal and illegal entry.

Republicans contend that the rising number of people attempting to cross the Southwest border have been inspired by Biden's early moves on immigration policy, which have included halting construction on the border wall and ending a program that forced asylum seekers to make their claims in Mexico and Central America.

"This administration's actions have had a direct cause and effect on this humanitarian and border crisis," said Rep. Michael McCaul, a Texas Republican.

The number of people caught attempting to cross the border has been rising since April and last month surpassed 100,000, the highest level since before the pandemic and on track to hit a 20-year high.

U.S. authorities are still turning most people away under a public health order issued at the start of the COVID-19 outbreak. But the Biden administration, reversing Trump, has decided to allow unaccompanied teens and children to enter the country to pursue claims for legal residency, either through asylum or for some other reason.

That has created a strain for federal authorities. Under a court order, the minors must be removed from the custody of U.S. Customs and Border Protection within 72 hours and then moved to shelters run by the Health and Human Services Department until a relative or other approved sponsor can claim them.

Homeland Security enlisted the Federal Emergency Management Agency to set up temporary facilities for several thousand minors, a decision that Republicans pointedly noted suggests a crisis.

"They deal with emergencies and they are now being deployed to the border and it's not an emergency," said Florida Rep. Kat Cammack. "Is that what I'm hearing?"

Mayorkas refused to give ground. He noted that Trump, despite his anti-immigration rhetoric and measures, faced a surge of migrants, as did President Barack Obama. The solution, he argued, is immigration legislation, which Biden supports, as well as support for Central American countries and improvements to the asylum process.

"It is a reflection of the fact that our system is broken," said the secretary, whose family brought him to the U.S. from Cuba as a child. He is the first refugee to lead Homeland Security.

Faced with questions about whether migrants are spreading COVID-19, Mayorkas said his department has implemented a policy that requires testing for anyone in Customs and Border Protection custody and quarantine for anyone with the virus. But he did not say when that started and he admitted that an unspecified number of migrants who could not be removed from the country, for reasons he did not make clear, were released into the United States before they were tested.

"We have addressed that situation," he said.

He also noted that Homeland Security has expanded an effort to vaccinate Border Patrol members. They have covered about 25% of frontline CBP personnel.

Mayorkas also appeared to acknowledge that some communities may not have been given adequate notice that they would be hosting one of the emergency shelters for migrant teens and children.

"If there was a failure to communicate with local officials with respect to our plans to open a facility in Midland, Texas, to shelter unaccompanied children, then that's a failure on our part and I'll follow up and make sure that doesn't happen again," he told Texas Rep. August Pfluger.

Texas Gov. Greg Abbott has complained that the apparent scramble to set up the location in Midland suggested an administration that was not ready or capable of handling the situation. "The Biden administration is completely not prepared for the number of children coming across this border," he said at a news conference.

The surge in migrant children has overwhelmed facilities and coincided with the arrival of immigrant families fleeing poverty and violence in Central America.

At a bus station Wednesday in Brownsville, Texas, in the Rio Grande Valley, dozens of immigrants recently released from government custody connected with a volunteer humanitarian organization to get legal services, clothing, water, food and toys for the children. About 50 people, including many children who held the hands of parents, waited to connect with the volunteer organization.

Naciel Marin of Nicaragua arrived last week at the border with her 15-month-old baby after a month-and-a-half journey to the U.S., where she was held for a few days before being released. She held the baby on her lap as they played with a toy xylophone that they were given, relieved to be in the U.S. before heading to Wisconsin to stay with in-laws. Marin said she would have made the trip to the U.S. regardless of who was president.

"Everything we've done is for the boy," she said of her baby, Matias.

During Wednesday's hearing before the House Homeland Security Committee, Mayorkas and some members of Congress attempted to shift the focus to non-border issues handled by his department. Those included the rise of domestic violent extremism, which the secretary said "now poses the most lethal and persistent terrorism-related threat" to the country, and the response to the suspected Russian hack of government computer networks.

But the hearing kept coming back to the border, where the new administration's policy is in sharp contrast to the preceding one.

Pressed on the "crisis" question, Mayorkas parried by reminding the committee of the Trump administration's decision to forcibly separate migrant children from their families as part of a zero-tolerance campaign, prompting national outrage.

"A crisis is when a nation is willing to rip a 9-year-old child out of the hands of his or her parent and separate that family to deter future migration," Mayorkas said. "That, to me, is a humanitarian crisis."

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Merchant reported from Houston.