WASHINGTON (AP) — The head of the Department of Homeland Security pushed back Wednesday against Republican criticism of the Biden administration's border policies, refusing to say that the surge of migrant children entering the United States is a “crisis.”

As GOP members of Congress argued that President Joe Biden had wrongly rolled back immigration policies enacted by former President Donald Trump, DHS Secretary Alejandro Mayorkas pointed to another Trump policy that was widely reviled: the separation of immigrant families under a “zero tolerance” policy on border crossings.

“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 told the House Homeland Security Committee. "That, to me, is a humanitarian crisis.”

The number of migrants being stopped at the U.S.-Mexico border has been rising since last April, and the administration is still rapidly sending back most single adults and families under a public health order issued by Trump at the start of the COVID-19 pandemic. But the U.S. is now allowing teens and children to stay, at least temporarily, and they have been coming in ever larger numbers. Border agents have also been prevented from expelling families with young children from Texas' Rio Grande Valley because of changes in Mexican policy.

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 officials spoke on condition of anonymity to discuss figures not yet publicly released.

The rise has left Biden in a difficult spot. He is criticized by Republicans for what they view as encouragement to illegal border crossings and by some Democrats over the prolonged detention of minors.

Rep. Jeff Van Drew, R-N.J., referenced a photo of a small crowd of demonstrators in Tijuana, Mexico, wearing matching T-shirts with the words “Biden, please let us in” that circulated widely on social media in recent days.

“I look at two months ago, and we had a certain situation. We were under control," Van Drew said. "And I look now with people crossing the border illegally in much larger numbers with shirts with our president’s name on it. That means something under anybody’s standard.”

The situation is also a challenge to Biden's effort to overhaul broader Trump policies intended to curtail both legal and illegal immigration.

“I can say quite clearly: Don't come over,” Biden said in an ABC News interview that aired Wednesday.

The number of migrants attempting to cross the border is at the highest level since March 2019. Mayorkas on Tuesday said it was on pace to hit a 20-year peak.

“The situation at the southwest border is difficult,” Mayorkas said 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.”

Under questioning Wednesday, Mayorkas and some Democrats also highlighted nonimmigration issues, including the threat of domestic extremism following the violent insurrection at the Capitol, while Republicans focused primarily on the border.

Several Republicans argued that Biden's rescinding of the Trump-era “Remain in Mexico” policy was a mistake. Small numbers of immigrants — far from the more than 70,000 people barred from entry under the policy — have been allowed into the U.S. during the first weeks of the new administration after they pass a COVID-19 screening. Human rights groups and lawyers have documented kidnappings, rapes and attacks in border cities against many migrants forced to wait in Mexico.

Republican Rep. Michael McCaul of Texas defended Trump's agreements to send people from Central America to other countries in the region instead of letting them seek asylum under American law.

“The Trump administration, in my judgment, did a masterful job in negotiating the ‘Remain in Mexico’ policy and the asylum cooperative agreements with Central America,” McCaul said. "It was actually a foreign policy achievement.”

Biden has moved to roll back those policies and restore a program allowing Central American children to seek asylum while in their home countries. Said Mayorkas, “Sometimes the tools of deterrence defy values and principles for which we all stand.”

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 people.

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

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Merchant report from Houston Associated Press writers Elliot Spagat in San Diego and Ron Nixon and Colleen Long in Washington contributed to this report.