WASHINGTON (Reuters) - U.S. President Donald Trump’s homeland security adviser, Tom Bossert, has resigned at the request of new national security adviser John Bolton, an administration official said on Tuesday, marking the latest departure from the White House of a senior adviser.

FILE PHOTO: Tom Bossert, homeland security adviser to President Donald Trump, holds a press briefing to publicly blame North Korea for unleashing the so-called WannaCry cyber attack at the White House in Washington, U.S., December 19, 2017. REUTERS/Kevin Lamarque

Bossert, who was a former deputy national security adviser to former President George W. Bush, had overseen the Trump administration’s response to the Hurricane Maria disaster in Puerto Rico, as well as cyber security policy. An official said Bolton, who started his new role on Monday, urged Bossert’s departure.

“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.

“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,” she said.

A source close to Bolton said other departures might be upcoming as the new national security adviser builds his own team in some sectors.

The potential 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.”

LONG LIST OF DEPARTURES

Bossert joins a long list of 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.

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

Bossert was socializing with U.S. intelligence officials and reporters at a conference on a coastal island off Georgia on Monday night, according to two people who were present.

One of the two people said Bossert gave no indication he was leaving the administration but joked at one point that he was unsure of his future “like everyone else who works for Trump.”

Bossert ran the administration’s work on cyber security issues and was considered a key voice for responding more aggressively to destructive cyber attacks launched by 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.

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