Jessica Guynn

USA TODAY

SAN FRANCISCO — Twitter CEO Jack Dorsey apologized for running an ad sponsored by a white supremacist group in violation of its own policies.

Dorsey blamed Twitter's automated system that allows Twitter accounts to place ads on the service.

"We made a mistake here and we apologize," he tweeted on Thursday.

Los Angeles musician and writer Ariana Lenarsky tweeted a screengrab of of the ad from a neo-Nazi white supremacist organisation called New Order that appeared in her Twitter timeline. The ad promoted an article: "The United States Was Founded as a White People’s Republic."

The ad appeared soon after Twitter said it would crack down on hate speech on the platform.

Twitter says the ad was on the service for less than an hour before it was removed and the account that placed it was suspended.

Twitter's hateful conduct policy prohibits content that “targets people on the basis of race, ethnicity, national origin, sexual orientation, gender, gender identity, religious affiliation, age, disability, or disease.”

The ad was an embarrassing gaffe for Twitter which earlier this week suspended high-profile accounts associated with the alt-right movement, a loosely organized group that, among other things, espouses white nationalism. The crackdown came on the heels of Twitter's pledge to combat hate speech.

"Our ads policy prohibits abusive content, hateful conduct, but our automated system missed one this past Monday," Twitter said in an emailed statement.

Twitter said it has identified how the ad slipped by its review system and is making "immediate changes to prevent this from happening in the future"

"Specifically, we're going to be adding more keywords and image recognition parameters to more quickly flag this type of content for review," the statement said.