The step is one of several that Twitter said it would take to crack down on white nationalists and other violent or hateful groups, which have become unwelcome on a service that once took an absolutist view of free speech.

Twitter (TWTR) said in a statement it would shut down accounts affiliated with non-government organizations that promote violence against civilians, and ban user names that constitute a violent threat or racial slur.

It said it would also remove tweets that it determined celebrate violence or glorify people who commit violence.

Twitter, a San Francisco company founded in 2006, had called itself “the free speech wing of the free speech party” and tried to stay out of battles among users. But that has changed as persistent harassers have driven some women and minorities off Twitter, limiting their ability to express themselves.

A rise in white nationalism in the United States has also changed tech industry standards. In August, social media networks began removing white nationalists after hundreds gathered in Charlottesville, Virginia, and one of them was charged with murdering a 32-year-old woman by running her down in a car.

In October, Twitter vowed to toughen rules on online sexual harassment, bullying, and other forms of misconduct.

Tweets can still include hate imagery, but users will have to click through a warning to see them, the company said. Hate images will be banned from profile photos, and further restricted where national laws require, as in Germany.

The Nazi swastika was the only specific example of a hateful image that Twitter gave, but the company said it would try to give warnings for all symbols historically associated with hate groups or which depict people as less than human.

Twitter said it had decided not to categorize the U.S. Confederate flag as hateful imagery, citing its place in history.