Democrats have condemned comments by the US president accusing half a dozen Democratic lawmakers of seditious behavior punishable by death.

The lawmakers — all veterans of the armed services and intelligence community — had called on U.S. military members to uphold the Constitution and defy “illegal orders” in a video message.

They also said the Trump administration “is pitting our uniformed military against American citizens”.

Although the lawmakers didn’t mention specific circumstances in the video, their message came as the Trump administration continues attempts at deployment of National Guard troops into U.S. cities for various roles.

President Trump’s initial comments were made on social media on Thursday.

On Friday in a Fox News Radio interview, he said “I’m not threatening them, but I would say they’re in serious trouble”.

Democratic leaders had warned the president's posts would encourage political violence.

House leader Hakeem Jeffries said Thursday, "We had patriotic members of the House and the Senate have their lives threatened by Donald Trump in the most unhinged, unacceptable, unconscionable, and un-American way.”

Pentagon spokesperson Sean Parnell challenged that illegal orders were being issued saying “We love the Constitution. These politicians are out of their minds,”.

One of the lawmakers, Rep. Chrissy Houlahan of Pennsylvania, said she has yet to hear from a Republican colleague expressing concern about Trump’s social media posts.

She said she was “dumbfounded by the silence” and viewed Trump’s remarks as a threat against her life and an explicit embrace of political violence.