US President Donald Trump was snarled in fresh controversy on Wednesday after he was accused of being disrespectful to a US soldier slain in an ambush in Niger, as well as showing insensitivity to the grieving family.

Trump moved quickly to dispute descriptions of his conversation with the pregnant widow of Sergeant La David Johnson, 25, who was one of four US service members killed in a jihadist ambush on October 4.

But two accounts of his condolence call suggest the president struggled to convey an empathetic tone.

“President Trump did disrespect my son and my daughter and also me and my husband,” Sergeant La David Johnson's mother, Cowanda Jones-Johnson, told the Washington Post.

She said she agreed with an earlier account of the call, given by a lawmaker who said she heard part of the conversation and accused Trump of insensitivity for telling Johnson's pregnant widow Myeshia Johnson that the young soldier “knew what he was signing up for”.

“I didn't hear the whole phone call, but I did hear him say, 'I'm sure he knew what he was signing up for, but it still hurts,'” Frederica Wilson, a Democratic congresswoman from Florida, told CNN.

Trump denied making the comments, tweeting Wednesday: “Democrat Congresswoman totally fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad!”

He later told reporters: “I did not say what she said,” and that “I had a very nice conversation”.

When asked about what proof he could offer, Trump said: “Let her make her statement again then you will find out.”

Johnson was among four US soldiers killed on October 4 in Niger, where militant Islamic State (IS) group fighters have established a presence.

The military has released few details about what happened, and on Tuesday Senator John McCain, who heads the Senate Armed Services Committee, said the Trump administration was not being forthcoming with information.

The Miami native's body was returned home Tuesday afternoon. His wife Myeshia, who is expecting the couple's third child, sobbed loudly while cradling his coffin after it was taken off a military plane.

After Trump's call, Myeshia “was crying, she broke down. And she said 'he didn't even know his name,'” Wilson said, referring to Trump and the dead soldier.