Facebook has unearthed a sophisticated plot that could be trying to influence the upcoming US elections.

Dozens of fake accounts are apparently being used to try to influence the result of November’s US midterm elections, the company reported. It said it was in the “very early” stages of its investigation and did not know exactly what the secretive group were trying to do, or how they were planning to do it.

More than 30 Facebook and Instagram pages were shut down because they were suspected of being part of the campaign, Facebook said in a blog post, because they were “involved in coordinated inauthentic behaviour”.

“This kind of behaviour is not allowed on Facebook because we don’t want people or organisations creating networks of accounts to mislead others about who they are, or what they’re doing,” it wrote.

The announcement comes as Facebook continues to be criticised for failing to protect the public from the kind of fake posts and disinformation that were spread in advance of the 2016 US presidential election and which Facebook has connected to the Russian state.

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The company does not know who was behind the co-ordinated attack, though it appears to be connected with protests planned in Washington next week.

The mysterious group uploaded nearly 10,000 posts and spent around $11,000 (£8,400) on adverts that ran across Facebook and Instagram. And it was planning to organise real events: a march against a far-right rally in Washington enlisted thousands of real supporters, and appeared to be expected to go ahead.

Whoever is behind the attack has gone to much more complex lengths to hide their identity than those involved in the previously reported work by the Russian Internet Research Agency (IRA), the company wrote.

They set up a whole range of different accounts – which together were followed by hundreds of thousands of people – which shared content under a range of different names. It is not clear what the group were planning to do with those popular accounts, some of which were explicitly political.

The most popular of the pages were named “Aztlan Warriors,” “Black Elevation,” “Mindful Being,” and “Resisters.”

An example of the pages that were run by the group (Facebook)

Facebook first became aware of the strange behaviour when it spotted the first unusual accounts two weeks ago, said Nathaniel Gleicher, its head of cybersecurity policy. They have now all been removed.

It said that finding the identities of the people behind the pages had been made more difficult because of security updates that Facebook has made over the last year, when the company has been repeatedly criticised for not doing enough to stop its platform being used to manipulate elections.

As such, Facebook might never be able to say where the attack was coming from, said Alex Stamos, Facebook’s chief security officer.