WASHINGTON - Department of Homeland Security Secretary Alejandro Mayorkas blamed the Trump administration Monday for the growing number of unaccompanied minors held in U.S. custody after crossing the Mexico border, saying the former president's policies left President Joe Biden with a "gutted" immigration system. 

A day after Trump blasted Biden's border policies in a speech to the Conservative Political Action Conference, Mayorkas appeared in the White House briefing room to respond and deny that the new administration is spiraling into a crisis.

"We are challenged at the border," Mayorkas said. "It is a stressful challenge."

"That is why, quite frankly, we are working as hard as we are, not only in addressing the urgency of the challenge, but also in building the capacity to manage it and to meet our humanitarian aspirations and execution of the president's vision," he said.

The influx of migrants at the border has become the most immediate issue for Biden as he delivers on campaign promises to reverse Trump's deterrent policies, including executive orders to curb deportations, freeze border wall construction and reunite families separated during Trump's "zero tolerance" crackdown in 2018.

When faced with skepticism about its more welcoming approach, given the difficulties faced by previous administrations along the Mexico border, Biden officials have depicted their efforts as an attempt to make amends for Trump policies.

While addressing reporters, Mayorkas announced the appointment of a longtime immigrant rights advocate, Michelle Brané, to direct a Biden task force responsible for contacting the parents of about 500 children separated under Trump, many of whom were deported to Central America.

Last week the Biden administration opened a tent facility in South Texas where the Department of Health and Human Services will shelter the soaring number of underage migrants crossing the border daily without a parent. Another tent site is planned for U.S. Customs and Border Protection's Del Rio sector in Texas, as the number of minors arriving without a parent has often surpassed 300 per day in recent week, a fourfold increase from October.

At the current rate, the Biden administration is on pace to receive even more unaccompanied minors in the coming month than during the 2019 crisis when record numbers of Central American children and families overwhelmed Trump officials.

Mayorkas told reporters the Biden administration is "working around-the-clock to replace the cruelty of the past administration with an orderly, humane and safe immigration process," citing Trump's border crackdown in 2018 that separated more than 3,000 children from their parents at the border.

"To put it succinctly, the prior administration dismantled our nation's immigration system in its entirety," he said. "When I started 27 days ago, I learned that we did not have the facilities available or equipped to administer the humanitarian laws that our Congress passed years ago. We did not have the personnel, policies, procedures or training to administer those laws. Quite frankly, the entire system was gutted."

In reality, the Biden administration has continued to rely on a Trump-era pandemic health order known as Title 42 as its primary border enforcement tool. The order allows U.S. border agents to bypass normal immigration laws, including asylum protections, to rapidly "expel" most unlawful border crossers back to Mexico.

Unlike Trump, Biden is not using Title 42 to expel teens and children. The policy has been denounced by immigrant advocates for sending vulnerable groups back to unsafe Mexican border cities where migrants are often targeted by gangs and criminals.

Biden continues to use Title 42 to sent back single adults and most families, but border arrests and detentions have surpassed 70,000 for the past four months in a row, one of the busiest periods of the past decade. Migrants from Central America and Mexico cite the economic fallout of the pandemic, a series of destructive hurricanes last fall, and deepening violence and corruption among the factors driving them to flee.

During his CPAC speech Sunday, Trump hammered Biden's border moves at length, telling the audience "he wants it all to go to hell."

"When I left office just six weeks ago, we had created the most secure border in U.S. history," Trump said, disregarding the rising number of illegal crossings recorded during his last several months in office.

"It took the new administration only a few weeks to turn this unprecedented accomplishment into self-inflicted humanitarian and national security disaster by recklessly eliminating our border, security measures, controls, all of the things that we put into place," Trump said.

Asked about specific Trump policies that have hampered the new administration, Mayorkas cited a contract DHS officials signed on the eve of Biden's inauguration with the union representing U.S. Immigration and Customs Enforcement employees.

The agreement, which Biden has canceled, would have given the union the right to "approve any policy changes in the immigration arena," Mayorkas said.

"I've been in government for almost 20 years now. I've never seen it a contract like that, and I certainly haven't seen a contract like that on the last day of an administration," he said.

"There can be honest disagreements about policy, but there has to be an overriding credo, and that is loyalty to our institutions and what they stand for and the fundamental rule of law," Mayorkas added. "And I find an agreement like that in dereliction of that duty."