* Domestic actors used similar tactics ahead of EU vote
* Social media platforms must share more data, EU says
* Brussels still debating regulation of online platforms (Adds FB comment)
By Alissa de Carbonnel
BRUSSELS, June 14 (Reuters) - Russian online outlets spread disinformation to sway voters in last month's European Union elections, the bloc said on Friday in a report calling for social media firms to take greater action or risk regulation.
The preliminary review of the European Parliament elections said there was evidence that both Russian and European online sources had sought to promote extreme views and polarise debate on divisive issues like migration and religion.
"The evidence collected revealed a continued and sustained disinformation activity by Russian sources aiming to suppress turnout and influence voter preferences," the report by the EU executive and its foreign service said.
Moscow has consistently denied it is targeting elections.
The EU report said some 1,000 cases were detected by a dedicated task force, whose staff was more than doubled to sixteen ahead of the vote. The unit monitors and fact-checks foreign news outlets but is not equipped with the tools of an intelligence agency to investigate cyber campaigns.
The 10-page report said there was no proof of "a distinct cross-border disinformation campaign from external sources specifically targeting the European elections."
Instead, EU officials cited examples of stories by Russian state-funded media - raising questions about how online content should be evaluated.
Some domestic political sources mimicked the tactics of Russian sources, the report said, and bots and fake accounts were used to amplify these efforts.
"It's not necessarily about breaking the law, but it is about attempting to mislead," Europe's Security Commissioner Julian King said.
EU leaders will discuss the review at a summit next week. A final report is due later in the year after which EU officials said they will consider further regulation of online platforms.
Despite voluntary steps taken by Facebook, Google and Twitter and other platforms, the review puts pressure on them to go further, including sharing data on their efforts.
In response, Facebook higlighted the steps it has taken into safeguarding the May ballot, such as partnering with local fact-checking organisations and dedicating staff to monitor the vote.
"The fight against false news will never be over," Facebook said in a statement. "We are making significant investments to remove fake accounts and clickbait and to promote high-quality journalism and news literacy."
Google and Twitter were not immediately available to comment.
The report called on them to develop tools to vet websites hosting ads, ramp up fact-checking and increase transparency by giving researchers access to data and governments information on the malign actors.
It said stricter rules governing paid-for political advertising implemented by Facebook should be improved and rolled out to upcoming national elections across the bloc. (Reporting by Alissa de Carbonnel Editing by Alexander Smith and Elaine Hardcastle)