|Mr Trump has been criticised for the alleged remarks|
US President Donald Trump says a claim that he made insensitive remarks to the recently bereaved widow of a soldier is "totally fabricated".
Congresswoman Frederica Wilson said he had told Myeshia Johnson: "He knew what he was signing up for, but I guess it hurts anyway."
The Democratic lawmaker said she was shocked by the alleged comments.
Sgt La David Johnson was among four US special service soldiers killed in Niger by Islamist militants this month.
Mr Trump had already been criticised for not contacting the families of the dead servicemen right after the fatal ambush on 4 October.
By Anthony Zurcher, senior North America reporter, BBC News
In US politics, nothing is off-limits any more.
After (inaccurately) swiping at his predecessors for not calling the family members of US soldiers killed in combat, Mr Trump is on the defensive over allegations he mishandled a call with a grieving widow.
The accuser is a partisan Democratic congresswoman and the president, not surprisingly, is pushing back hard. This controversy is spiralling towards the gutter.
Mr Trump made this bed, however. He was quick to cite the slain son of chief of staff John Kelly to justify his contention that Barack Obama didn't always make phone calls. Then there were the disparaging comments candidate Mr Trump made last summer about the parents of a Muslim-American soldier killed in Iraq.
The more this story drags on - and it will drag on - the more damage it could do to a president who wraps himself in the symbols of patriotism and the military, but is in danger of being viewed by the public as lacking empathy when it counts most.
An important presidential role is consoler-in-chief during times of tragedy. Successful politicians learn early that they need a human touch.
It's a job Mr Trump, the anti-politician, has little experience doing - and it shows.
How did Trump respond?
The president tweeted on Wednesday morning: "Democrat Congresswoman Totally Fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad!"
Mr Trump has yet to provide any evidence.
A White House official said Mr Trump's conversations with the families of dead servicemen were private.
Mr Trump later told reporters: "I did not say what she [Ms Wilson] said... 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."
What do his accusers say?
Ms Wilson, who represents a Florida district, told CNN the president's call had been made shortly before Sgt Johnson's coffin arrived by aircraft in Miami.
Ms Wilson told WPLG, a Miami TV station, she had heard the president's "so insensitive" remarks to the widow on speakerphone in a limousine.