WASHINGTON (NewsNation Now) — 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 that some people were released without being tested for COVID-19, though a new testing policy has been implemented.

But Mayorkas 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.”

Homeland Security Secretary Alejandro Mayorkas testified before a House panel in his first appearance before Congress since his confirmation.

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 migrants being stopped at the U.S.-Mexico border has been rising since last April, and the administration is still rapidly expelling most single adults and families under a public health order issued by President Donald Trump at the start of the COVID-19 pandemic. But it is allowing teens and children to stay, at least temporarily, and they have been coming in ever larger numbers.

More than 4,000 migrant children were being held by the Border Patrol as of Sunday, including at least 3,000 in custody longer than the 72-hour limit set by a court order, according to a U.S. official. The agency took in an additional 561 on Monday, twice the recent average, according to a second official. Both spoke on condition of anonymity to discuss figures not yet publicly released.

Biden is facing pressure from both sides. Republicans are criticizing the administration for what they view as encouragement to illegal border crossers, while some Democrats are concerned with the prolonged detention of minors.

It also presents a challenge to his administration’s effort to overhaul the broader policies, under his predecessor former President Donald Trump, that sought to curtail both legal and illegal immigration.

The number of migrants attempting to cross the border is at the highest level since March 2019, with Homeland Security Secretary Alejandro Mayorkas warning that it is on pace to hit a 20-year peak.

“The situation at the southwest border is difficult,” Mayorkas conceded Tuesday in his most extensive remarks to date on the subject. “We are working around the clock to manage it and we will continue to do so. That is our job.”

The number of children crossing by themselves, mostly from Central America, appears to be surging in particular in the Rio Grande Valley of South Texas. The Border Patrol took in 280 there alone on Monday.

The total of 561 unaccompanied minors from Monday offers a snapshot of how quickly conditions have changed along the border. That was up 60% from the daily average in February, one of the officials said. In May 2019, during the last surge, the one-day peak was 370 teens and children.

Children and teens crossing by themselves rose 60% from this January to more than 9,400 in February, according to the most recent statistics released publicly by U.S. Customs and Border Protection.

The Health and Human Services Department plans to open shelter facilities at Moffett Federal Airfield near San Francisco and in Pecos, Texas, to handle the flow. It is also looking to expand a facility in Donna, Texas, in the Rio Grande Valley, to hold 2,000.

Also, the Dallas Convention Center is scheduled to begin holding children as early as Wednesday with plans to accommodate up to 3,000. Another makeshift holding center in Midland, Texas, that opened last weekend for 700 children had 485 on Monday.

Mayorkas noted that Trump, despite his anti-immigration rhetoric and measures, faced a surge of migrants as did former president Barack Obama. The solution, he argued, is immigration legislation, which Biden supports, 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.

Some of the increase in adults is due to people who are repeatedly caught after being expelled under the public health order issued last year to help prevent the spread of COVID-19. Other factors include economic upheaval caused by the pandemic and recent hurricanes that worsened living conditions in Central America. Officials say it’s also likely that smugglers have encouraged people to try to cross under the new administration.

Mayorkas said the a surge in the number of children is a challenge for the Border Patrol and other agencies amid the coronavirus pandemic. But he rejected a Trump-era policy of sending them immediately back to Mexico or other countries.

“They are vulnerable children and we have ended the prior administration’s practice of expelling them,” he said.

Though there have been previous migrant surges, including under Trump, Republicans in Congress say that Biden’s support for new immigration legislation and his decision to allow people to make legal asylum claims have become a magnet for migrants.

House Minority Leader Kevin McCarthy led a delegation of a dozen Republican lawmakers on Monday to the border in Texas and blamed the Biden administration for driving an increase in migrants by actions that include halting border wall construction and supporting legislation in Congress that would provide a path to citizenship for millions of undocumented people now in the country.

“The sad part about all of this is it didn’t have to happen. This crisis was created by the presidential policies of this new administration,” McCarthy said.

Biden pushed back in an interview with ABC’s George Stephanopoulos, noting previous surges under Trump and pointing out that his administration has been trying to discourage people from crossing while it works to restore an asylum system undermined by his predecessor. “I heard the other day that they’re coming because they know I’m a nice guy. Yeah, well, here’s the deal. They’re not.”

Trump confronted a similar surge in 2019 even as he rushed to expand the border wall system and forced people seeking asylum to do so in Central America or remain in Mexico. A year earlier, he forcibly separated migrant children from their families as part of a zero-tolerance campaign that became one of the most significant political challenges of his administration.

The Biden administration is allowing migrants who are under 18 years old and cross by themselves to remain in the country while the government decides whether they have a legal claim to residency, either under asylum law or for some other reason.

Mayorkas noted that 80% of the minors, most of whom are from the three Northern Triangle countries of Central America, have relatives in the U.S. and 40% have a parent. “These are children being reunited with their families who will care for them,” he said.

The Biden administration last week ended a Trump policy that made relatives reluctant to contact HHS to retrieve children for fear of being deported themselves.

Besides setting up new temporary facilities to house migrant children, it is also backing aid to Honduras, Guatemala and El Salvador to try to stem the flow of migrants at the source.

Mayorkas also criticized the previous administration for dismantling an asylum system that would have enabled a more “orderly” immigration system, cutting aid to Central America and failing to vaccinate Border Patrol agents.

“We have to rebuild the entire system, including the policies and procedures required to administer the asylum laws that Congress passed long ago,” Mayorkas said.

Also, he said the Biden administration is working to make the asylum process shorter and to make it possible to petition from an applicant’s home country rather than make a dangerous and uncertain journey.

“We have no illusions about how hard it is, and we know it will take time,” Mayorkas said. “We will get it done.”

The Associated Press contributed to this report. Most reporting by AP’s Ben Fox, Elliot Spagat and Nomaan Merchant.