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(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.

“Although the people behind this activity attempted to conceal their identities, our investigation found links to individuals associated with the Chinese government,” Nathaniel Gleicher, Facebook’s head of cybersecurity policy, wrote in a blog post.

Beijing responded on Tuesday. Geng Shuang, a spokesman for China’s Ministry of Foreign Affairs, told reporters that “people will have their own judgments about what is happening in Hong Kong, and what the truth is.”

“I think it is reasonable that Chinese media use overseas social media to communicate with local people, to tell stories about China, to introduce Chinese policies,” he said.

This is the first time tech companies have pointed the finger at Beijing for covert efforts to influence messaging around the Hong Kong protests. Pro-democracy demonstrators have massed throughout Hong Kong for 11 consecutive weekends.

An account that was titled “Dream News” tweeted about the demonstrators, “We don’t want you radical people in Hong Kong. Just get out of here!”

The accounts posted in multiple languages and at least some of them appeared to target American audiences. One account, that was set up more than 10 years ago, said in its Twitter bio, “Conservative News from the USA and Abroad. #Catholic Defender of the Constitution of the United States.” The account claimed to support President Donald Trump.

Other accounts also purported to be operated by people in the United States, listing locations like Chicago and Long Beach, California.

Twitter initially identified the network and shared details about the accounts with Facebook (FB). Then, Facebook (FB) identified about a dozen pages, accounts and groups that were tied to the operation.

The Facebook pages were followed by about 15,000 accounts, the company said.

Twitter (TWTR) said many of the accounts accessed its platform using virtual private networks because Twitter (TWTR) is blocked in China.

China, of course, is not alone in using social media to spread unrest. Covert campaigns have been tied to Russia and Iran, among other countries.

In 2018 the US Department of Justice brought charges against a Kremlin-linked troll group that posed as American on social media in the run-up to the 2016 US presidential election.

-— Joshua Berlinger, Steven Jiang and Nanlin Fang contributed to this report.