FIRST ON FOX: Arizona Attorney General Mark Brnovich has written to President Biden urging him to replace Homeland Security Secretary Alejandro Mayorkas, describing the secretary’s tenure as an "unmitigated disaster" for U.S. border security – amid an increasing migrant surge.

"Mr. President, Secretary Mayorkas’ short tenure at DHS has been nothing short of an unmitigated disaster for border security, and for American safety and sovereignty," Brnovich wrote in a letter Friday to Biden. "For this reason, I am requesting you replace him with a Secretary who is committed to the rule of law and enforcement of border policies that protect American communities."

MAYORKAS ANNOUNCES 212,672 MIGRANT ENCOUNTERS IN JULY, SAYS BORDER ‘IS ONE OF THE TOUGHEST CHALLENGES WE FACE’ 

The letter comes days after Mayorkas announced that there were more than 212,000 migrant encounters in July alone, a 13% increase over June and a massive increase over the 40,929 in July 2020.

While he noted that nearly half of those encounters led to an expulsion under Title 42 public health protections, Mayorkas called the situation "one of the toughest challenges we face."

"It is complicated, changing and involves vulnerable people at a time of a global pandemic," he said.

Additionally, while he noted that numbers had been rising since April 2020, "the increase is most certainly sharper over the last several months and greater than in June."

Privately, he was more dour in his assessment. Speaking to Border Patrol agents the same day, in audio obtained by Fox News, Mayorkas called the situation "unsustainable."

MAYORKAS SAYS BORDER CRISIS ‘UNSUSTAINABLE’ AND ‘WE’RE GOING TO LOSE' IN LEAKED AUDIO 

"The other day I was in Mexico, and I said, if our border is our first line of defense, we're going to lose and this is unsustainable," Mayorkas said. "We can't continue like this, our people in the field cant continue and our system isn't built for it."

It has coincided with increasing pressure on Mayorkas from Republicans. Rep. Andy Biggs, R-Ariz., introduced articles of impeachment against Mayorkas last week, while Sen. Tom Cotton, R-Ga., has called for Mayorkas to resign.

Brnovich, in his letter to Biden, described a chaotic scene on the ground in border states and elsewhere.

"Every day under your administration, international cartels operate along our border with near impunity. They ferry migrants across the border to create distractions and gaps for our Border Patrol agents," he said. "They smuggle illicit and deadly drugs to countless American communities to wreak havoc in unsuspecting school systems and families."

GOP DEMANDS BIDEN ADMINISTRATION ACT AFTER MAYORKAS CALLS BORDER CRISIS ‘UNSUSTAINABLE’ 

The Biden administration has blamed root causes, like poverty, corruption and violence in Central America, for the surge. Mayorkas also said the "resurgence" of the American economy is partly responsible, while conceding that the ending of Trump-era policies also had an effect.

"Another reason is the end of the cruel policies of the past administration and the restoration of the rule of laws of this country that Congress has passed, including our asylum laws that provide humanitarian relief," he said on Thursday.

However, Mayorkas has claimed that a plan that involves tackling root causes, going after smugglers and opening up legal asylum pathways will ultimately prevail. The administration has also extended Title 42 and ramped up flights into Mexico and Central America to return rejected migrants.

"We have a plan, we are executing our plan and that takes time," he said in Texas.

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But Republicans have zoned in on the Biden administration’s ending of Trump-era policies and what they see as a refusal of the Biden administration to enforce immigration and border laws.

"Americans are counting on your administration to enforce our laws and to stop the crisis at the border," Brnovich said. "Please Mr. President. Do the right thing. Enforce our laws. Push back against the lawless cartel activity that is ripping apart communities along our border. Protect Americans and our country’s sovereignty. And surround yourself with men and women who will uphold the constitutional oaths of office."