U.S. President Donald Trump tweeted on Sunday that Nielsen "will be leaving her position and I would like to thank her for her service."

Nielsen was appointed to the post on Dec. 6, 2017. It is not clear what was the immediate reason for her resignation.

The New York Times reported that Trump had been unhappy with Nielsen as the number of illegal immigrants entering the United States had risen and he blamed her for not doing enough to secure the border.

U.S. news outlet CNN cited a government official as saying that Nielsen found her position "untenable" due to Trump's requests, while CBS News said Nielsen's tenure "has at times been rocky."

"Despite our progress in reforming homeland security for a new age, I have determined that it is the right time for me to step aside. I hope the next secretary will have the support of Congress and the courts," Nielsen said in her resignation letter.

In another tweet, Trump announced that current U.S. Customs and Border Protection Commissioner Kevin McAleenan, 47, will become acting secretary for the department. "I have confidence that Kevin will do a great job!" Trump tweeted.

Nielsen has supported the Trump administration's tough stance on illegal immigrants, and has overseen controversial policies where immigrant children were separated from their parents for extended periods during detention and trials.

At the end of March, Trump criticized Mexico and several Central American nations for not halting illegal immigrants heading north to cross the U.S. border, and later threatened to close the nation's southern border with Mexico.

"Mexico is doing NOTHING to help stop the flow of illegal immigrants to our Country. They are all talk and no action," Trump tweeted.

"Honduras, Guatemala and El Salvador have taken our money for years, and do Nothing," he added.

The president's remarks stood in contrast to those of Nielsen, who earlier has thanked Honduras, Guatemala and El Salvador for their efforts to help the United States secure the border.