"Brazen forgeries" designed to fan sectarian violence in Northern Ireland were sown online by a "Russian operation", a US think tank has said.

One involved a fake tweet by the UK defence minister about the Real Irish Republican Army (RIRA), a militant group.

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Another one was a false email by a Northern Irish party leader on Brexit.

And a third one said the RIRA had invited "Muslim extremists" to join them to "fight on the [sic] Irish soil".

The inflammatory content was put online between March 2018 and April 2019, according to Ben Nimmo, a scholar of Russian propaganda at the Atlantic Council, a think tank in Washington.

The "suspected Russian intelligence operation" sought to "fan divisions between, and within, Western countries", he said in a report published on Saturday (22 June).

"The Russian operation launched not one, but at least three separate forgeries, one attributed to a senior minister, another to a party leader. Each time, the apparent purpose was to exacerbate the already raw divides in Ireland and the United Kingdom," he added.

The fake tweet by then British defence minister Gavin Williamson had said RIRA was involved in the attempted murder of former Russian spy Sergei Skripal in England.

The false email by Democratic Unionist Party (DUP) leader Arlene Foster said she was "favourable" to an EU deal on Brexit which was a red rag to unionist militants.

A related Russian story warned of "bloody confrontation" in Northern Ireland if the DUP backed the deal.

The purported invitation to jihadists by an RIRA "headhunter" was an untraceable screenshot of an online Muslim discussion board.

Some of the content was posted by "ostensibly Irish personas" on US social media giant Facebook.

Other parts were published on US aggregator website Reddit and American image sharing website Imgur.

The same or similar material was also posted, often simultaneously, in smaller online forums in English, French, German, Spanish, and Ukrainian, the Atlantic Council study showed.

"Each of these stories used the same techniques. They started by posting a forgery online, amplified it with a number of accounts created especially for that purpose, and finally boosted the content on Facebook," the think tank's Nimmo said.

One tell-tale sign of the forgeries was use of "distinctly non-native" syntax and phrases, the expert noted.

The operation tried to cover its tracks by using "burner accounts" - fake online personas which only ever posted one item - he also said.

The one on the British defence minister used a purloined photo of British actor Hugh Laurie as its profile picture, the study showed.

The Good Friday Agreement ended decades of sectarian violence in Northern Ireland in 1998.

Discussion on Northern Ireland's status after the UK leaves the EU has raised tension, amid the spectre of a return to border security checkpoints.

But "none of the [forged] stories gained any traction" Nimmo said.

"In part, this was likely due to the nature of the content. In part, it was due to the obsessive secrecy with which the operation was conducted," he added.

"The operation never had the chance to build up a following or develop its audience," he said.

For its part, Facebook took down the suspicious accounts in a broader clean-up in May, after the US firm identified what it called "co-ordinated inauthentic behaviour" that was "emanating from Russia".

A spokeswoman for the Russian embassy in Dublin told The Irish Times newspaper that the Atlantic Council study had "nothing to do with reality".

But Nimmo told the newspaper that "the combination of Brexit, a hard border, and all the rest of it is an open wound".

"If you are a hostile foreign actor, why would you not stick your finger in it?," he said.