WASHINGTON — The Pentagon on Monday said it is reviewing "serious allegations of misconduct" against US Democratic Senator Mark Kelly.

Kelly, a retired US Navy captain, was one of six Democratic lawmakers who released a video urging military members to refuse unlawful orders.

The announcement in a post on the Department of War X account did not detail the allegations against Kelly, but suggested the move is in response to Kelly's statements: "All servicemembers are reminded that they have a legal obligation under the UCMJ to obey lawful orders and that orders are presumed to be lawful."

Kelly said he learned of the investigation via the Pentagon's social media post on Monday.

"If this is meant to intimidate me and other members of Congress from doing our jobs and holding this administration accountable, it won't work," Kelly responded on X.

In addition to the Pentagon, US President Donald Trump posted on social media last week accusing Kelly and other Democratic lawmakers of "seditious behavior, punishable by death".

"I've given too much to this country to be silenced by bullies who care more about their own power than protecting the Constitution."

Kelly was a combat pilot in the US Navy who served during the first Gulf War and later flew four space missions for Nasa from 2001 to 2011. He is married to former Arizona congresswoman Gabrielle Giffords, who retired after being shot during a 2011 mass shooting.

Even though he retired from the military, he is still subject to the Uniform Code of Military Justice (UCMJ) — a federal law enacted by Congress in 1951 that subjects members of the military to a special set of rules.

It applies to all active-duty members as well as activated National Guard and Reserve members, military academy students and some civilians serving in support of the military during wartime.

It also applies to retirees and gives the military the ability to recall retirees for applicable offences, such as disobeying federal laws that "prohibit actions intended to interfere with the loyalty, morale, or good order and discipline of the armed forces".

The Pentagon said on Monday that further action may be taken against Kelly, including a recall to active duty for court-martial or administrative measures.

In the statement, the Pentagon said the matter would be "handled in compliance with military law, ensuring due process and impartiality", and that further official comment would be limited "to preserve the integrity of the proceedings".

"All servicemembers are reminded that they have a legal obligation under the UCMJ to obey lawful orders and that orders are presumed to be lawful. A servicemember's personal philosophy does not justify or excuse the disobedience of an otherwise lawful order."

The video by Democrats, all of whom have served in the military or intelligence community, was shared by Michigan Senator Elissa Slotkin and features Kelly and US Representatives Chris Deluzio, Maggie Goodlander, Chrissy Houlahan and Jason Crow. In it, they tell military members they do not need to obey illegal commands.

Kelly says in the video: "Our laws are clear. You can refuse illegal orders."

"This administration is pitting our uniformed military and intelligence community professionals against American citizens," the video says.

"Like us, you all swore an oath to protect and defend this constitution. Right now, the threats coming to our constitution aren't just coming from abroad but from right here at home."

Trump reacted to the video in a series of irate Truth Social posts on Thursday, accusing the lawmakers of "SEDITIOUS BEHAVIOR AT THE HIGHEST LEVEL".

"Each one of these traitors to our Country should be ARRESTED AND PUT ON TRIAL. Their words cannot be allowed to stand," he wrote. "An example MUST BE SET."

His remarks were met with bipartisan condemnation and the president — who is the commander-in-chief of armed forces — walked back those statements in a Fox interview on Friday, clarifying that he was "not threatening death" against the Democratic lawmakers.

Reposting the Pentagon statement on X on Monday, defence secretary Pete Hegseth called the Democrats' video "despicable, reckless, and false," referring to the lawmakers as the "Seditious Six".

He said that encouraging US warriors to ignore their commanders' orders "undermines every aspect of 'good order and discipline'".

"Their foolish screed sows doubt and confusion — which only puts our warriors in danger," he wrote, singling out Kelly as the only one of the six who is still subject to the UCMJ.

"As was announced, the Department is reviewing his statements and actions, which were addressed directly to all troops while explicitly using his rank and service affiliation—lending the appearance of authority to his words," Hegseth wrote.

He added that Kelly's conduct "brings discredit upon the armed forces and will be addressed appropriately". — BBC