Washington DC - The US military on Monday said it was considering a court-martial against Democratic senator and former top astronaut Mark Kelly for appearing in a video urging troops to refuse illegal orders.

The Pentagon's probe marks an extraordinary escalation in the Trump administration's backlash against six Democrats with military or intelligence service backgrounds who made the viral video.

Kelly, a senator in Arizona, was a decorated Navy combat pilot and went to space on several missions, including as commander of Space Shuttle Endeavour's final flight.

In a statement on social media, the Pentagon threatened to call Kelly, who is retired from the military, back to active duty in order to face a court-martial under the Uniform Code of Military Justice.

Officials indicated that the senator may have undermined the "loyalty, morale, or good order and discipline of the armed forces," pointing to laws barring interference in military cohesion.

"A thorough review of these allegations has been initiated to determine further actions, which may include recall to active duty for court-martial proceedings or administrative measures," the Pentagon said.

It is highly unusual for the Pentagon – long allergic to overt politics until President Donald Trump's return to power – to publicly threaten a sitting member of Congress.

The video posted on social media Friday called on the military to "refuse illegal orders" and featured Kelly alongside Michigan's Elissa Slotkin, along with Jason Crow of Colorado, Chris Deluzio and Chrissy Houlahan of Pennsylvania, and Maggie Goodlander of New Hampshire.

Trump initially accused the group of "seditious behavior, punishable by death." Over the weekend, he posted a new statement calling for jail terms.

Democrats slammed the president's comments as "absolutely vile" threats against the six senators and representatives.

Trump took to social media on Saturday night, writing in an all-caps rant that "the traitors that told the military to disobey my orders should be in jail right now, not roaming the fake news networks trying to explain that what they said was ok."

The Democrats did not specify the orders to which they were referring, but Trump has ordered the National Guard into multiple US cities, in many cases against the wishes of local officials, in a bid to bring alleged rampant unrest under control.

Abroad, Trump has ordered strikes on a series of supposed drug-smuggling vessels in the Caribbean Sea and eastern Pacific Ocean that have left more than 80 people dead and which experts say are illegal.

Kelly did not immediately respond to a request for comment.