In an unexpected move, President Donald Trump's homeland security adviser, Tom Bossert, is resigning, the White House announced Tuesday, and the reason was unclear.

“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,” Press secretary Sarah Sanders said in a statement.

The White House did not specify when Bossert's resignation will take effect but the announced departure comes the day after the president's new national security adviser, John Bolton, started in his role and comes against the backdrop of a series of high-level departures from the Trump administration in recent weeks.

It also comes two days after National Security Council spokesman Michael Anton resigned from the administration. The president called Anton on Sunday to thank him for his service, according to a senior White House official, and Sanders praised him as “one of the smartest and most talented individuals I’ve ever worked with.”

On Sunday, Bossert sat down exclusively with ABC News' Martha Raddatz for an interview on "This Week," in which he defended the president's decision to send National Guard troops to the southern border with Mexico to support U.S. Border Patrol agents and said that all options are on the table for a potential response to the suspected chemical attack in Syria.

“I wouldn’t take anything off the table,” Bossert told Raddatz. “These are horrible photos [from the alleged attack]. We’re looking into the attack at this point.”