The announcement comes days after Twitter and Facebook said they dismantled a similar campaign originating in mainland China.

"This discovery was consistent with recent observations and actions related to China announced by Facebook and Twitter," said Shane Huntley, one of Google's security leaders, in a blog post. But he stopped short of identifying the origin of the channels.

Twitter and Facebook on Monday said the channels they removed had engaged in a state-backed effort by China to undermine the protests in Hong Kong through posts calling participants dangerous and vile extremists.

The protests, which have presented one of the biggest challenges for Chinese President Xi Jinping since he came to power in 2012, began in June as opposition to a now-suspended bill that would allow suspects to be extradited to mainland China for trial in Communist Party-controlled courts.

They have since swelled into wider calls for democracy.

"We are deeply concerned by Chinese attempts to manipulate public opinion by spreading disinformation about the situation in Hong Kong," a United States State Department spokeswoman told Reuters.

In Beijing, Foreign Ministry spokesman Geng Shuang declined direct comment on YouTube's decision, but said the Chinese people's greatest wish was for the chaos and violence to end in Hong Kong.

"The will of 1.4 billion people cannot be blocked or controlled, and of course cannot be shut out," he told reporters.

The Chinese mission to the United Nations sent Reuters a link to a story from the ruling Communist Party media outlet People's Daily that said Twitter and Facebook "abused media freedom" in cracking down on accounts that had revealed violence in the protests.

Australian Associated Press