WASHINGTON - One of US Defence Secretary Pete Hegseth's leading advisers, Mr Dan Caldwell, was escorted from the Pentagon on April 15 after being identified during an investigation into leaks at the Department of Defence, a US official told Reuters.

Mr Caldwell was placed on administrative leave for "an unauthorised disclosure," the official said, speaking on condition of anonymity.

The decision has not been previously reported.

"The investigation remains ongoing," the official said, without providing details about the nature of the alleged disclosure, including whether it was made to a journalist or to someone else.

President Donald Trump's administration has moved to aggressively pursue leaks, an effort that has been enthusiastically embraced by Mr Hegseth at the Pentagon.

A March 21 memo signed by Mr Hegseth's chief of staff, Mr Joe Kasper, requested an investigation into "recent unauthorised disclosures of national security information involving sensitive communications."

Mr Kasper's memo left open the possibility of a polygraph, although it was unclear if Mr Caldwell was subjected to one.

Although Mr Caldwell is not as well known as other senior Pentagon officials, he has played a critical role as an adviser to Mr Hegseth.

His importance was underscored in a leaked text chain on Signal disclosed by The Atlantic in March.

In it, Mr Hegseth named Mr Caldwell as the best staff point of contact for the National Security Council as it prepared for the launch of strikes against the Houthis in Yemen.

Mr Caldwell had drawn attention in Washington for past views that critics have called isolationist, but which advocates said sought to right-size America's defence priorities.

A Marine Corps veteran who deployed to Iraq, Mr Caldwell was quoted saying before going to the Pentagon that America would have been better off if US troops had just stayed home.

"I think the Iraq war was a monstrous crime," Mr Caldwell told the Financial Times in December 2024.

He was also a sceptic of US military assistance to Ukraine and advocated for US retrenchment from Europe.

The decision to put Mr Caldwell on administrative leave is separate from a wave of firings since Mr Hegseth, a former Fox News host and combat veteran, took over the Pentagon in January.

Those firings of top brass have included the chairman of...