EU officials said Friday that 'Russian sources' stepped up a long-running disinformation campaign against the European Union in a bid to undermine the bloc's elections last month.

The sources used fake social media accounts, bots and news sites to amplify existing divisive content by targeting particular groups of voters and countries, Security Commissioner Julian King said.

'The number of disinformation cases attributed to Russian sources ... doubled as compared to the same period a year ago,' King said, highlighting a report by European Commission investigators.

Julian King, the EU's Security Commissioner, said there were 1,000 cases of disinformation linked to Russia in May - when elections were held - compared to 400 this time last year

'So almost 1,000, as compared with over 400.'

The report from the 28-nation EU's executive arm follows pre-election warnings from Brussels to EU countries and social media giants like Facebook and Twitter to guard against 'fake news' from Moscow.

'Available evidence has not allowed us to identify a distinct cross-border disinformation campaign from external sources specifically targeting the European elections,' the report said.

'However, the evidence collected revealed a continued and sustained disinformation activity by Russian sources aiming to suppress turnout and influence voter preferences,' the report said.

When asked whether the report contradicted itself, King replied that disinformation aimed at dividing Europeans was 'increasingly locally focused' rather than large scale.

He added that EU steps to counter disinformation may have also had 'some sort of deterrent effect.'

Justice Commissioner Vera Jourova told the same news conference that there was 'no big bang moment' - like Facebook's Cambridge Analytica scandal - to draw attention to organised manipulation.

Then, after revelations from a whistleblower, it was shown that tens of millions of users had their personal data hijacked by Cambridge Analytica, a political firm working for Donald Trump in 2016.

But King cited examples of what he said were false reports spread by Russian sources, such as the claims that the EU had made Poland poorer than when it had been a communist country.

King accused Russia of using a network of bots, fake social media accounts and fake news sites to spread and amplify what he called 'divisive content'

The British commissioner also said Russian sources, without identifying them, suggested French President Emmanuel Macron wants to expel some countries from the EU.

He also said a fake Twitter account allegedly linked to Russia has spread false information that the EU has Nazi roots.

The Commission report said that 'political actors' in member states often adopted tactics and story lines used by Russian sources to damage the EU.

For example, 'malicious actors' used the fire at the Notre-Dame Cathedral in the French capital two months ago to showcase what they see as an alleged decline in Western Christian values.

The alleged trolls also blamed Austria's political crisis and government collapse on the so-called 'European deep state,' as well as 'German and Spanish security services' and others, the report said.

The commission said it will take time to determine the 'scope and impact' of disinformation campaigns because of their complex and sophisticated nature.

Official figures showed that turnout for the May 23 to 26 elections was at nearly 51 percent, a 20-year high, amid growing debate about the future of the EU.

And, even though populists and eurosceptics made significant inroads, they did not achieve the breakthrough feared by the political establishment.

Mainstream parties still control the lion's share of the 751-seat European Parliament.

The Commission welcomed a voluntary campaign begun last year by US tech giants Google, Twitter and Facebook to stop the spread of falsehoods and hatred on their platforms.

The tech giants took down millions of fake accounts and did more to screen political ads, Jourova said.

But King and Jourova, a politician from the Czech Republic, said they must provide more detailed information to help identify malicious actors and the countries targeted.

They said the new commission that will be formed in the wake of the elections could still pass laws affecting the tech giants if the voluntary approach is insufficient.