U.S. President Donald Trump told the widow of a soldier killed in an ambush in Niger that her husband "knew what he signed up for," according to a Florida member of Congress who says she heard part of the conversation on speakerphone.

Rep. Frederica Wilson said she was in the car with Myeshia Johnson on Tuesday on the way to Miami International Airport, where the body of Johnson's husband, Sgt. La David Johnson, was arriving, when Trump called.

When asked by Miami station WPLG if she indeed heard Trump say that, she answered: "Yeah, he said that. To me, that is something that you can say in a conversation, but you shouldn't say that to a grieving widow." She added: "That's so insensitive."

La David Johnson was among four servicemen killed in the African nation of Niger earlier this month. Militants thought to have been affiliated with ISIS ambushed them while they were patrolling in unarmoured trucks with Nigerien troops.

Wilson, a Democrat, said she did not hear the entire conversation and Myeshia Johnson told her she couldn't remember everything that was said when asked it about it later.

On Wednesday morning, Trump tweeted that Wilson had "totally fabricated" the conversation and he had 'proof.'

Wilson quickly shot back on CNN, saying "the president evidently is lying, because what I said is true." 

The serviceman's mother, Cowanda Jones-Johnson, told The Associated Press on Wednesday that the Congress representative's account was correct. She said "not only did he disrespect my son," but also the family.

The back and forth over Trump's remarks continued later in the morning on his way to a meeting at the White House. 

I had a very nice conversation with the woman, the wife, who sounded like a lovely woman. Did not say what the congresswoman said."

When reporters asked the president about his earlier tweet, Trump said: "Let her (Wilson) make her statement again and you'll find out."

Trump has been criticized for not reaching out right away to relatives of the four killed in Niger.

On Monday, Trump said he had written letters that had not yet been mailed. His aides said they had been awaiting information before proceeding.

With files from CBC News