President Donald Trump's homeland security adviser, Tom Bossert, has been fired by Trump's new national security adviser, John Bolton, according to Bloomberg.

Bossert, who advised the president on terrorism, cybersecurity, and natural-disaster response, has been a stalwart defender of Trump over the past 15 months.

President Donald Trump's homeland security adviser, Tom Bossert, has been fired from his White House position, Bloomberg's Jennifer Jacobs reported on Tuesday.

"The president is grateful for Tom's commitment to the safety and security of our great country," the White House press secretary, Sarah Huckabee Sanders, said in a statement confirming Bossert's departure. "President Trump thanks him for his patriotic service and wishes him well."

The news emerged the day after John Bolton took over as the president's third national security adviser. Jacobs reported that Bolton unexpectedly asked for Bossert's resignation.

Bossert worked closely with Bolton's predecessor, Lt. Gen. H.R. McMaster, and is reportedly a close ally of the White House chief of staff, John Kelly. He was widely liked in the White House and seen as a "cheerleader and mentor type to many," CNN's Jeff Zeleny reported.

Once a deputy homeland security adviser to President George W. Bush, Bossert was a stalwart defender of Trump's since joining the administration during the presidential transition, serving as a trusted adviser on terrorism, cybersecurity issues, and natural disasters.

Bossert has been criticized for defending the president's controversial comments about issues as varied as white supremacist protesters in Charlottesville, Virginia, last summer and Russian interference in the 2016 election.

On Sunday, Bossert asserted that "no voter in this country was influenced" by Russian bots during the election.

"I honestly do believe that there is a large degree of misunderstanding in the reporting and there's a whole lot of attempted politicking going on meant to try to undermine the legitimacy of the election," he told NBC News.

Some believe that Bolton, a controversial former US ambassador to the UN and foreign-policy hawk, will push several other top national security staffers out as he asserts control over the office and brings in a new team. Michael Anton, the National Security Council spokesman, resigned two days ago.