United States President Donald Trump has denied a Congresswoman’s claim that he told a soldier’s grieving – and pregnant – widow on her way to meet her husband’s casket that “he knew what he signed up for”.

But if Mr Trump’s denial was designed to get the Congresswoman to reverse her version of events or the tide of public opinion flowing against him, it had the opposite effect.

“This man is a sick man,” Democrat Congresswoman Frederica Wilson (Florida) told CNN when informed of the president’s denial, adding: “He’s cold-hearted and he feels no pity or sympathy for anyone.”

In a tweet posted Wednesday night (AEST), Mr Trump said: “Democrat Congresswoman totally fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad!”

It was unclear what “the proof” was, but the claim again raised suspicions that Mr Trump tapes his phone conversations.

Congresswoman Wilson was riding in a limousine with the soldier’s widow, Myeshia Johnson, and her family to meet his casket at Miami International Airport when she overheard the President’s call and what she described on Twitter as his “insensitive comments”.

“Basically he said, ‘Well I guess he knew what he signed up for, but I guess it still hurts’,” Ms Wilson told CNN. “I heard what he said because the phone was on speaker.”

Sergeant La David Johnson, 25, of Miami Gardens, Florida, was killed in Niger earlier this month during an ambush carried out by Boko Haram and other extremists linked to Islamic State.

The fallen soldier’s mother backed Ms Wilson’s account, telling the Washington Post she and her husband were also in the vehicle when the president rang and he “did disrespect my son and daughter and also me and my husband”.

Democrat Congresswoman totally fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad!

— Donald J. Trump (@realDonaldTrump) October 18, 2017

The Florida Representative said Ms Johnson was already distraught, particularly after hearing the casket would be closed.

Ms Wilson said that when Ms Johnson hung up from speaking with Mr Trump, “she looked at me and said, ‘he didn’t even know his name’.

“Now, that’s the worst part,” Congresswoman Wilson said.

Johnson’s body arrived home on Tuesday, nearly two weeks after his death on October 4.

He was one of four US soldiers killed when a joint patrol of American and Niger forces was ambushed by militants believed linked to the Islamic State group.

Bryan Black, La David Johnson, Jeremiah Johnson and Dustin Wright were killed while serving in Niger. Photo: US Department of Defense

“This is a young woman who has two young children, who is six months’ pregnant with her third child [and] she has just lost her husband.

“She was just told that he cannot have an open-casket funeral which gives her all kinds of nightmares of how his body must look, how his face must look. And this is what the United States President says to her?

“That is not something you say to a grieving wife.”

Ms Wilson said Ms Johnson could not say anything in response to the President because she was crying.

“There were other family members in the car and they were all crying,” Ms Wilson said.

“All she said when it was time to hang up was ‘thank you, goodbye’.”

The congresswoman’s reaction to the call was to take the phone and “curse him out”, but she was not handed the phone.

When the limousine arrived at the airport to greet the casket, a sobbing Ms Johnson and her two children awaited the casket, alongside various local officials and law enforcement officers, who were on site to pay their respects.

Ms Johnson lay across the US flag draping her husband’s casket, sobbing uncontrollably.

Niger ambush could be Trump’s Benghazi, Wilson says

Ms Wilson said there are still questions about Sergeant Johnson’s death that are yet to be answered.

“[The family] don’t know why he was separated from the other soldiers,” she said.

“This could turn out to be another Benghazi, and I have asked for an investigation.

“We want to find out exactly what happened and I’m expecting a classified briefing when I return to Washington to answer some of these mitigating questions that I can’t answer on my own.”

Ms Wilson said Sergeant Johnson, who she mentored in her dropout prevention program for boys which she founded in 1993, was a hero in their community.

“There was no reason for the President to be so insensitive, not only to the family of this soldier, but the impervious rhetoric is disrespectful to the family of every soldier that has paid the ultimate price of freedom,” she said.

Trump boasts he calls families where Obama did not

The call came after the US President falsely claimed his predecessor, Barack Obama, did not contact the families of US troops killed in the line of duty.

The claim drew a swift response from Mr Obama’s foreign policy adviser Ben Rhodes, who said it was “an outrageous and disrespectful lie”.

This is an outrageous and disrespectful lie even by Trump standards. Also: Obama never attacked a Gold Star family. https://t.co/JgzTUIzWIa

— Ben Rhodes (@brhodes) October 16, 2017

The President made the claims in response to questions about why he had not publicly spoken about the killing of the US soldiers in Niger two weeks ago.

“If you look at President Obama and other presidents, most of them didn’t make calls,” Mr Trump said during a news conference in the Rose Garden with the Senate majority leader, Mitch McConnell.

“A lot of them didn’t make calls. I like to call when it’s appropriate.”

It's shameful how @realDonaldTrump politicizes issue of fallen soldiers to deflect criticism for not recognizing the 4 who died in Niger!

— Rep. Frederica Wilson (@RepWilson) October 17, 2017

A former staffer of Barack Obama’s office said the claims were a lie. Photo: AP