US President Donald Trump's homeland security adviser, Tom Bossert, has resigned, the president's spokeswoman says, in the latest departure from the White House of a senior adviser.
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 in his post 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 spokeswoman Sarah Sanders said in a statement on Tuesday.
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"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," Sanders said.
Bolton's arrival at the White House also prompted the departure of Trump's national security council spokesman, Michael Anton.
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 HR 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 cyber security 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 cyber security experts, who viewed him as a knowledgeable voice in the room.
Rob Joyce, the White House's cyber security tsar, who reported to Bossert, is still working in the administration, a White House official said.
