Washington (AFP) - A group of white Confederate flag supporters in the US state of Georgia have been charged under an anti-gang law for disrupting an African-American birthday party with "terroristic threats," officials said Monday.

Fifteen members of the group were indicted for driving vehicles bearing Confederate flags to the celebration, where a confrontation ensued, Douglas County District Attorney Brian Fortner said in a statement.

The document, which was posted online by the Southern Poverty Law Center (SPLC), alleges that members of the "Respect the Flag" group violated the state's Street Gang Terrorism and Prevention Act.

The charges, which were handed down Friday and also posted by SPLC, refer to the group as "a criminal street gang," and allege that it unlawfully participated "in criminal gang activity, specifically terroristic threats."

The birthday party host, Melissa Alford, said that a string of pickup trucks fluttering Confederate flags disrupted the July 25 event in Douglasville, just west of Atlanta, with threats and racial epithets, according to SPLC.

One of the truck's drivers, Levi Bush, told the Atlanta Journal-Constitution that the vehicles were leaving a nearby event and passed by the birthday haphazardly, when attendees began yelling and throwing rocks.

The indictment alleges that the 15 individuals acted in a manner to "unlawfully threaten to commit a crime of violence to persons attending a party.. with the purpose of terrorizing those individuals and in reckless disregard for the risk of causing such terror."

Morris Dees, who founded SPLC, said that he was unaware of any instance in which an anti-gang law had been used to prosecute such a group, according to The New York Times.

"I don't know why it hasn't been used before," he said.

The Confederate flag, an emblem of the once slave-owning South that is seen by many now as a white supremacist symbol, has become a focus of an intense debate on racism in America since nine black worshippers were killed at a Charleston church in June.

The suspect in the massacre, Dylann Roof, had posted a picture of himself holding the Confederate flag and a handgun.