President Donald Trump is reportedly fuming at Department of Homeland Security Secretary Kirstjen Nielsen over a rise in illegal border crossings in 2018 following a historic and frequently-touted low in 2017. Their acrimonious relationship reportedly prompted Nielsen to draft a letter of resignation, the New York Times reports.

The president also expressed anger Nielsen and DHS weren't doing more to take advantage of the Department of Justice's new “zero tolerance” immigration policy, which legalizes the separation of families caught crossing the border. According to the New York Times, Trump berated Nielsen at a cabinet meeting on Wednesday, prompting Nielsen to contemplate resigning.

Trump was reportedly upset that Nielsen didn't support his plan to separate families who cross the border illegally. On Thursday, Nielsen supported and justified the policy in an interview with NPR, arguing U.S. citizens are subject to the same treatment.

"That's no different than what we do every day in every part of the United States — when an adult of a family commits a crime," she told NPR. "If you as a parent break into a house, you will be incarcerated by police and thereby separated from your family."

"Illegal aliens should not get just different rights because they happen to be illegal aliens," she added.

Attorney General Jeff Sessions announced Monday that the Department of Homeland Security will refer every single person crossing the U.S. southwestern border illegally to the Department of Justice to be prosecuted, and that, as a policy, families will be separated.

“These actions are necessary,” Sessions said. “And they are made even more necessary by the massive increases in illegal crossings in recent months. This February saw 55 percent more border apprehensions than last February. This March saw triple the number from last March. April saw triple the number last April. “

Immigration advocates say the policy is inhumane and unnecessary, especially because the number of border crossings has been so historically low.