Donald Trump has denied a claim that he told the widow of a US soldier killed in an ambush in Niger that her husband "knew what he signed up for".

Congresswoman Frederica Wilson said she was in the car with Myeshia Johnson on the way to Miami International Airport to meet the body of Mrs Johnson's husband, Sergeant La David Johnson, when the president called.

Asked by Miami station WPLG if she heard Mr Trump say that, Ms Wilson 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. That's so insensitive."

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

He did not specify what proof he had.

Sgt Johnson was among four servicemen killed in the African nation of Niger earlier this month when militants thought to be affiliated with the Islamic State group ambushed them while they were patrolling with Nigerian troops in unarmoured trucks.

Ms Wilson said she did not hear the entire conversation and Mrs Johnson told her she could not remember everything that was said when asked it about it later.

Mr Trump has been criticised for not reaching out straight away to relatives of the four killed in Niger.

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

The incident came after Mr Trump provoked a row by suggesting his predecessors as president failed to meet families of military personnel killed in war.

"Most of them didn't make calls," he said of his predecessors.

He said it was possible that Mr Obama "did sometimes" but "other presidents did not call".

The record is plain that presidents reached out to families of the dead and to the wounded, often with their presence as well as by letter and phone.

George W Bush called or met "hundreds, if not thousands" of family members of the war dead, and even at the height of two wars, "wrote all the families of the fallen", said his spokesman Freddy Ford.

Barack Obama's official photographer, Pete Souza, tweeted that he photographed Mr Obama "meeting with hundreds of wounded soldiers, and family members of those killed in action".