Donald Trump's Homeland Security adviser, Tom Bossert, has resigned, the U.S. president's spokesperson says — the latest in a string of senior advisers to leave the White House.

An administration official said Bossert, a former deputy national security adviser to President George W. Bush, had left at the request of Trump's new national security adviser, John Bolton, who began working at the White House on Monday.

"The president is grateful for Tom's commitment to the safety and security of our great country," White House spokesperson Sarah Huckabee Sanders said in a statement.

"Tom led the White House's efforts to protect the homeland from terrorist threats, strengthen our cyber defences and respond to an unprecedented series of natural disasters."

Bolton's arrival at the White House also prompted the departure of Trump's national security council spokesperson, Michael Anton.

'Huge mistake'

The partial purge raised concern from Democratic Senator Chris Coons, a member of the Senate foreign relations committee, who told CNN that Bolton "seems to be swiftly moving to eliminate or to move toward an early retirement several of the president's advisers."

Jamil Jaffer, a former chief counsel to the Senate foreign relations committee and associate counsel to Bush, said it was a "huge mistake" to force Bossert out.

"Tom is a very smart and highly skilled national security leader who has been a beacon of principle, capability and discipline in an otherwise chaotic White House," he said in a statement. "Letting Bossert go at a time of heightened threats, and when there is significant churn on the overall national security team, is a yet another unforced error."

Bossert joins a long list of other senior officials who have resigned or been fired since Trump took office in January 2017, including previous national security advisers Michael Flynn and H.R. McMaster, White House chief of staff Reince Priebus, communications directors Hope Hicks and Anthony Scaramucci, economic adviser Gary Cohn and chief strategist Steve Bannon.

Secretary of State Rex Tillerson, Health Secretary Tom Price and Veterans Affairs Secretary David Shulkin have also left.

Bossert oversaw the administration's work on cybersecurity issues and was considered a key voice for responding more aggressively to destructive cyber attacks launched by hostile adversaries, including Russia, Iran and North Korea.

He helped guide the administration's decisions in recent months to blame and impose costs on each of those countries in an effort to create a more forceful cyber-deterrence strategy.

Bossert was generally well respected by cybersecurity experts, who viewed him as a knowledgeable voice in the room.

Rob Joyce, the White House's cybersecurity czar, who reported to Bossert, is still working in the administration, a White House official said.