US President Donald Trump has said a lawmaker's claim he made a soldier's widow cry is "totally fabricated".
Congresswoman Frederica Wilson said she was shocked by the president's comments to the bereaved wife of a fallen soldier.
The Democratic lawmaker claimed he told the widow: "He knew what he was signing up for, but I guess it hurts anyway."
Sgt La David Johnson was among four US special service troops killed in Niger by Islamist militants this month.
Mr Trump has already been criticised for not contacting the families of the dead servicemen right after the fatal ambush on 4 October.
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 did not immediately provide the evidence.
Ms Wilson, who represents a Florida district, told CNN that the president's call was made shortly before Sgt Johnson's coffin arrived in Miami.
"This gentleman has a brain disorder," said the lawmaker, "and he needs to be checked out."
Ms Wilson told WPLG, a Miami TV station, she heard the president's "so insensitive" remarks to the widow on speakerphone in a limousine.
"Yeah, he [President Trump] said that," Ms Wilson said. "To me, that is something that you can say in a conversation, but you shouldn't say that to a grieving widow.
"And everyone knows when you go to war, you could possibly not come back alive. But you don't remind a grieving widow of that."
Ms Wilson told the Washington Post that the widow, Myeshia Johnson, who is expecting the couple's third child, broke down in tears after the conversation.
"He made her cry," Ms Wilson said.
The congresswoman told the newspaper that she wanted to grab the phone and "curse him out".
But an army sergeant who was holding the handset would not let her speak to the president, she said.
A White House official said Mr Trump's conversations with the families of fallen servicemen were private.
The full context of the conversation is not known. Ms Wilson said that when she had asked Ms Johnson about the exchange, she said she could not remember.
The alleged remarks sparked angry comments on social media, with Ms Wilson saying on Twitter that Mr Trump did "not possess the character, empathy or grace to be president of the United States".
Mr Trump has been on the defensive over the Niger deaths since a reporter asked him at the White House on Monday why he had yet not called the families.
He provoked fury by falsely claiming that his predecessor, Barack Obama, and other former US presidents did not call the relatives of fallen service members.
Mr Trump also said he had written letters to the families of the four killed in Niger and planned to call them soon.
The White House later said the president had spoken to the families, but did not say when.
On Tuesday, Mr Trump ratcheted up the row by suggesting that President Obama did not call the family of Mr Trump's chief of staff Gen John Kelly when his son was killed in Afghanistan in 2010.
This is not the first time Mr Trump has found himself in an imbroglio over US veterans.
As presidential candidate, he mocked Senator John McCain for having been captured as a prisoner of war in Vietnam.
He also engaged in a racially charged feud with the parents of decorated army captain Humayun Khan, who was killed in Iraq in 2004.