President Donald Trump says Homeland Security Secretary Kirstjen Nielsen is leaving her job amid growing frustration by the administration over the number of southern border crossings.

Trump said in a tweet on Sunday that U.S. Customs and Border Protection Commissioner Kevin McAleenan will become the acting head of the sprawling department of 240,000 people. McAleenan is a longtime border officer, reflecting Trump's priority for the department initially founded to combat terrorism after the Sept. 11 attacks.

Two people familiar with the matter said Nielsen went to the White House to speak with Trump on Sunday following their trip to the border. The people say she has long been frustrated by the difficulty getting other departments to help with the growing number of families coming crossing the border.