© Provided by CNET Twitter's ad tool reportedly allowed advertisers to target users interested in neo-Nazis and other hate groups. Jaap Arriens/NurPhoto via Getty Images

Twitter , under mounting criticism it's not doing enough to purge its platform of hate speech, is apologizing for an apparent gap in its rules that allowed advertisements to target neo-Nazis and other hate groups on the platform.

Twitter already has a policy that bars hateful conduct, including promoting violence or directly attacking people based on race, religion, sexual orientation and other characteristics. But the BBC reported Wednesday it discovered a flaw in the platform that made it possible for ads to be directed to users who posted about or searched for words such as "transphobic," "white supremacists" and "anti-gay."

© Jaap Arriens/NurPhoto via Getty Images Twitter's ad tool reportedly allowed advertisers to target users interested in neo-Nazis and other hate groups.

Twitter tools let advertisers direct specific ads at a customers based on their interests and activity on the service, including keywords they use. The list of keywords is supposed to be restricted, but the BBC's use of the tool found that a search for the term "neo-Nazi" indicated it had a potential audience in the UK of 67,000 to 81,000 users.

A campaign tapping "islamophobes," "islamophobia," "islamophobic" and the hashtag "#islamophobic" had a potential reach of as many as 114,000 Twitter users, the BBC found.

Twitter didn't immediately respond to CNET's request for comment but told the BBC that existing policies to prevent abuse of keyword targeting weren't correctly applied.

"[Our] preventative measures include banning certain sensitive or discriminatory terms, which we update on a continuous basis," Twitter said in a statement to the BBC. "In this instance, some of these terms were permitted for targeting purposes. This was an error.

"We're very sorry this happened and as soon as we were made aware of the issue, we rectified it," Twitter said.

In November, about a dozen civil rights activists gathered outside Twitter's headquarters in San Francisco to demand the company ban white supremacists from the platform. Activists delivered petitions, signed by 110,000 people, that urged Twitter to ban white supremacists.

Twitter isn't the first social network to face criticism for the ads it allows. In 2019, the Los Angeles Times reported that on Facebook, advertisers were able to target people interested in perpetrators of the Holocaust, including Joseph Goebbels, Josef Mengele and Heinrich Himmler.