Pete Hegseth, the US defense secretary, told soldiers under his command in Iraq to ignore legal advice about when they were permitted to kill enemy combatants under their rules of engagement.

The anecdote is contained in a book Hegseth wrote last year in which he also repeatedly railed against the constraints placed on “American warfighters” by the laws of war and the Geneva conventions.

Hegseth is currently under scrutiny for a 2 September attack on a boat purportedly carrying drugs in the Caribbean, where survivors of a first strike on the vessel were reportedly killed in a second strike following a verbal order from Hegseth to “kill everybody”.

Hegseth has denied giving the order and retained the support of Donald Trump. The US president said Hegseth told him “he did not say that, and I believe him, 100%”. But some US senators have raised the possibility that the US war secretary committed a war crime.

In the book, The War on Warriors, Hegseth relates a story about a legal briefing at the beginning of his service in Iraq, in which he told the men under his command to ignore guidance from a military judge advocate general’s (JAG) attorney’s guidance about the rules of engagement in the conflict.