New York (CNN Business) Twitter will no longer accept advertisements from state-controlled media outlets, the company announced Monday, hours after it joined with Facebook to take down a covert Chinese social media campaign that sought to undermine pro-democracy protests in Hong Kong.

"We want to protect healthy discourse and open conversation," Twitter said in a statement posted on its website.

Twitter said the new policy would only apply to "news media entities that are either financially or editorially controlled by the state." While state media cannot pay for advertisements that would amplify their message, they will be allowed to continue posting on the platform. Twitter said the new restrictions do not apply to outlets funded by taxpayers, or independent public broadcasters.

The new policy was announced shortly after Twitter said it had identified a network of more than 900 accounts originating in China that "were deliberately and specifically attempting to sow political discord in Hong Kong, including undermining the legitimacy and political positions of the protest movement." Some accounts called protesters "cockroaches" or compared them to Islamic State terrorists. The offending accounts have been taken down from Twitter and Facebook.

It also came after BuzzFeed News and others reported that media outlets in China, most of which are funded by the state and tightly controlled by authorities in Beijing, had been buying advertisements on Facebook and Twitter that portrayed the protests negatively.

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