A top Democratic lawmaker with a military background has reacted strongly to US defense secretary Pete Hegseth’s call for “no quarter” for US enemies during a Friday press briefing at the Pentagon, calling such an order – if followed by troops – a potential violation of international law.

The US senator Mark Kelly, of Arizona, posted on Friday on X that “‘No quarter’ isn’t some wanna be tough guy line – it means something. An order to give no quarter would mean to take no prisoners and kill them instead.”

Kelly added: “That would violate the law of armed conflict. It would be an illegal order. It would also put American service members at greater risk. Pete Hegseth should know better than to throw around terms like this.”

According to a transcript of the briefing, Hegseth said: “We will keep pressing, keep pushing, keep advancing – no quarter, no mercy for our enemy.”

Critics of Hegseth say the phrase “no quarter” is more than a belligerent figure of speech, implying that enemy combatants will not be taken prisoner but instead executed. Under the Hague Convention of 1899, that is considered a war crime.

An amendment to the convention, from 1907, states that “it is especially forbidden … to declare that no quarter will be given”.

According to the International Committee of the Red Cross (ICRC), global “humanitarian law prohibits the use of this procedure, that is, ordering that there shall be no survivors, threatening the adversary therewith, or conducting hostilities on this basis”.

The ICRC’s International Humanitarian Law Databases says that under the statute of the international criminal court, “declaring that no quarter will be given” is a war crime in international armed conflicts.

Hegseth’s use of the phrase came amid a wide-ranging briefing during which he claimed Iran’s new supreme leader, Mojtaba Khamenei, is wounded and likely disfigured. The defense secretary also questioned Khamenei’s ability to govern.

“We know the new so-called not-so-supreme leader is wounded and likely disfigured,” Hegseth said. “He put out a statement yesterday. A weak one, actually, but there was no voice and there was no video. It was a written statement.”

An Iranian official told Reuters on Wednesday that Khamenei’s injuries were light. On Friday, Iran’s ambassador to Japan, Peyman Saadat, said Khamenei had not been “impaired”.

Kelly’s criticism of Hegseth’s remark comes amid an ongoing war of words between the two veterans that has spilled into the courtroom.

In November, Kelly and five other Democratic lawmakers appeared in a video in which they urged troops to disobey unlawful military directives from Donald Trump’s administration.

The president accused the lawmakers of sedition “punishable by DEATH” in a social media post. And Hegseth called for Kelly’s demotion from the senator’s retired rank of captain.

The Pentagon subsequently began investigating Kelly, citing a federal law that allows retired service members to be recalled to active duty on orders of the defense secretary for possible court-martial.

But a judge ruled in February that he knew of no federal supreme court precedent to justify the Pentagon censuring of a US senator and appeared skeptical of arguments made by a government attorney, asking if they weren’t “a bit of a stretch”.