The finger-and-thumb gesture - which is also a popular emoji - is being used by some as a "sincere expression of white supremacy", according to the Anti-Defamation League (ADL).

But the US anti-hate group says the "overwhelming usage" of the hand gesture today is still to show approval or that someone is OK.

Therefore "particular care must be taken not to jump to conclusions about the intent behind someone who has used the gesture".

Other symbols added to the list include burning neo-Nazi symbols, images of the "Happy Merchant" and the slogan "Diversity = White Genocide".

 Members of the far-right group Proud Boys made the hand gesture at a rally in Oregon

The Anti-Defamation League started its "Hate on Display" list in 2000 with the aim of helping people recognise signs of extremism.

It now has over 200 entries, including the swastika and Ku Klux Klan's burning cross.

"Even as extremists continue to use symbols that may be years or decades old, they regularly create new symbols, memes and slogans to express their hateful sentiments," said ADL boss Jonathan Greenblatt.

"We believe law enforcement and the public needs to be fully informed about the meaning of these images, which can serve as a first warning sign to the presence of haters in a community or school."

 It's thought the OK sign was first used in Britain in the 17th century

The ADL says the OK symbol has become a "popular trolling tactic" from "right-leaning individuals, who often post photos to social media of themselves posing while making the gesture".

It started out as an online joke on 4Chan - taking an innocent gesture and pretending there was a hidden meaning behind it, hoping to trick the media and left-leaning people into outrage.

But the joke was so successful and widespread among the far-right, that many believe the OK sign is changing meaning.

The man accused of killing 51 people at mosques in Christchurch, New Zealand, earlier this year flashed the OK hand sign when he was in court.

He's pleaded not guilty to murder.

The suspect flashed a hand sign as he appeared in court charged with murder

Dr Paul Stocker, a historian who specialises in far-right movements, says the OK sign is a way of people who are in the far-right to communicate to each other.

"It's a coded message to people who are in the know and understand what the far-right are doing," he tells Radio 1 Newsbeat.

"That symbol indicates to core supporters that they are one of them."

The writer says it's common for extremist groups to take signs "already in the public domain and twist their meaning".

"The process of finding symbols has accelerated with the alt-right. They operate mainly online and use coded language and memes that might seem innocuous, but if you drill down into them they have meaning."

The Anti-Defamation League is very keen to stress that more often that not, the symbol means everything is OK.

"The overwhelming usage of the hand gesture today is still its traditional purpose.

"As a result, someone who uses the symbol cannot be assumed to be using the symbol in either a trolling or white supremacist context."