Gabbard, who met Assad in 2017, has previously drawn criticism for voicing skepticism that his regime was behind the chemical weapons attacks on civilian people.
"I think that the evidence needs to be gathered and, as I have said before, if there is evidence that he has committed war crimes, he should be prosecuted as such," Gabbard said at a CNN town hall on Sunday.
Asked if she is unsure whether Assad is a war criminal, Gabbard said, "Everything that I have said requires that we take action based on evidence. (If) the evidence is there, there should be accountability".
"I served in a war in Iraq, a war that was launched based on lies, and a war that was launched without evidence. And so the American people were duped," she said.
"So as a soldier, as an American, as a member of Congress, it is my duty and my responsibility to exercise skepticism any time anyone tries to send our service members into harm's way or use our military to go in and start a new war," she added.
She has become the first-ever Hindu to be running for the presidency in the US.
Gabbard, who converted to Hinduism early in her life, is popular among Indian-Americans. If elected, she would be the youngest ever and first woman President of the United States. She would also be the first non-Christian and first Hindu to occupy the top post.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)