Calley maintains that he was following orders from his superior
The US army officer convicted for his part in the notorious My Lai massacre during the Vietnam War has offered his first public apology, a US report says.
"There is not a day that goes by that I do not feel remorse for what happened," Lt William Calley was quoted as saying by the Columbus Ledger-Enquirer.
He was addressing a small group at a community club in Columbus, Georgia.
Calley, 66, was convicted on 22 counts of murder for the 1968 massacre of 500 men, women and children in Vietnam.
"I feel remorse for the Vietnamese who were killed, for their families, for the American soldiers involved and their families. I am very sorry," the former US platoon commander said on Wednesday.
The My Lai massacre was a turning point in the Vietnam War
He was sentenced to life in prison for his role in the killings in 1971. Then-US President Richard Nixon commuted his sentence to three years' house arrest.
But Calley insisted that he was only following orders, the paper reported.
He broke his silence after accepting a friend's invitation to speak at the weekly meeting of the Kiwanis Club, a US-based global voluntary organisation.
At the time of the killings, the US soldiers had been on a "search and destroy" mission to root out communist fighters in what was fertile Viet Cong territory.
Although the enemy was nowhere to be seen, the US soldiers of Charlie Company rounded up unarmed civilians and gunned them down.
When the story of My Lai was exposed, more than a year later, it tarnished the name of the US army and proved to be a turning point for public opinion about the Vietnam War.