The senior Democrat involved in a war of words with Donald Trump over a phone call to the wife of a slain US soldier has said the White House is full of "white supremacists".

Frederica Wilson, who is a member of the House of Representatives for Florida, became embroiled in a war of words with Trump administration when she recounted a phone call the president made to the widow of a special forces soldier killed in Niger.

The president's chief of staff John Kelly hit back by accusing the congresswoman of taking credit for a new FBI building in 2015.

The retired four-star general said Wilson bragged about how she lobbied President Barack Obama for $20m to fund a new FBI building in south Florida.

Video footage exonerated Wilson of the charge but in an interview with the New York Times, she said the clash was tinged with racial overtones.

"The White House itself is full of white supremacists," she said. "I feel very sorry for him because he feels such a need to lie on me and I'm not even his enemy.

"I just can't even imagine why he would fabricate something like that. That is absolutely insane. I'm just flabbergasted because it's very easy to trace."

"They are making themselves look like fools. They have no credibility. They are trying to assassinate my character, and they are assassinating their own because everything they say is coming out and shown to be a lie."

Trump said accusations Kelly was lying were "sickening" and described his right hand man as "a very elegant man." In a tweet he later called Wilson "wacky":

The argument over what Trump said to the widow Army Sgt. La David Johnson came as former presidents Bush and Obama used keynote speeches to launch thinly-veiled attacks on the conduct of the current occupant of the Oval Office.