United States President Donald Trump has denied telling a war widow on her way to meet her husband's casket that the soldier "knew what he signed up for".

Key points: Sgt La David Johnson and three other soldiers were killed in a terrorist ambush in Niger on October 4

Sgt La David Johnson and three other soldiers were killed in a terrorist ambush in Niger on October 4 Mr Trump had not addressed soldiers' deaths, criticised Obama and others for not calling families of death soldiers

Mr Trump had not addressed soldiers' deaths, criticised Obama and others for not calling families of death soldiers Congresswoman who heard President's call to grieving widow weeks after death says call was insensitive

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 ISIS.

Congresswoman Frederica Wilson was riding in a limousine with the soldier's widow, Myeshia Johnson, and her family to meet the 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."

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

Mr Trump says Ms Wilson's claim is fabricated but is yet to provide evidence.

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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. ( Supplied: 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," Ms Wilson said.

"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.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 16 seconds 1 m 16 s Body of Special Forces soldier killed in Niger arrives in US (Image Reuters: Courtesy Aaron J. Jenne/ US Air Force)

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 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, others 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".

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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."

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