President Donald Trump's homeland security advisor, Tom Bossert, is stepping down, the White House announced Tuesday.

The administration did not give a reason for Bossert's departure, which came as a surprise while the president pushes to ramp up border security.

A White House official, speaking on condition of anonymity, told CNBC that the move "seems like a natural turnover with a new [National Security Council] director." John Bolton, a foreign policy hard-liner, started as Trump's national security advisor Monday, succeeding H.R. McMaster.

The official said that they were not aware of any points of difference between Bolton and Bossert.

Still, the action indicates Bolton is moving to put his own stamp on White House personnel. When a CNBC reporter pointed out that Bossert was seen as one of the most effective people in the Trump administration, another White House official, also speaking on the condition of anonymity, responded: "That was likely the problem."

Press secretary Sarah Huckabee Sanders released a statement on Bossert's resignation:

"The President is grateful for Tom's commitment to the safety and security of our great country. Tom led the White House's efforts to protect the homeland from terrorist threats, strengthen our cyber defenses, and respond to an unprecedented series of natural disasters. President Trump thanks him for his patriotic service and wishes him well."

Bossert's resignation came two days after he took to the television airwaves to defend Trump's decision to send National Guard troops to the border with Mexico.

"We've got a leaking boat on our border. And we're all quibbling with how much water's in the boat and how fast we're bailing it out," Bossert told ABC's "This Week" on Sunday. "I think at this point the president's been pretty clear, enough is enough, fix the actual problem and stop that leak."

Bossert also served in the administration of President George W. Bush and worked as a homeland security advisor during Trump's transition period before taking office.

Last year, Bossert was reportedly tricked into giving up his personal email address by a prankster pretending to be Jared Kushner.