WASHINGTON, April 7 (Xinhua) -- U.S. President Donald Trump Sunday tweeted that Secretary of Homeland Security Kirstjen Nielsen "will be leaving her position."

Current U.S. Customs and Border Protection Commissioner Kevin McAleenan will become acting secretary for the department, Trump said in another tweet.

The announcement came after U.S. news outlets reported that Nielsen was set to hand in her resignation letter Sunday evening at a meeting with Trump.

"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.

It is not clear what was the immediate reason for Nielsen's resignation. U.S. news outlet CNN cited a government official as saying that Nielsen found it "untenable" on the position due to requests from Trump.

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

Nielsen's tenure "has at times been rocky," said CBS News.

Nielsen, 46, was appointed to the post on Dec. 6, 2017, after her predecessor John Kelly was elevated to be the White House's chief of staff.

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

McAleenan, 47, served as the Commissioner of U.S. Customs and Border Protection since Jan. 20, 2017.