The campaign of the French presidential frontrunner, Emmanuel Macron, has been targeted by hackers linked to Russia, according to researchers with a Japanese anti-virus firm.

The researchers added to previous suggestions that the centrist politician was being singled out for electronic eavesdropping by the Kremlin.

On Monday, Mounir Mahjoubi, digital chief for the Macron campaign, confirmed there had been attempted intrusions but said they had all been thwarted. “It’s serious, but nothing was compromised,” he said.

Macron faces his rival, the far-right Marine Le Pen in France’s presidential runoff on 7 May. Macron favours a strong EU, while Le Pen wants to pull France out of the bloc.

The Tokyo-based firm, Trend Micro, said it had made the discovery by monitoring the creation of rogue, lookalike websites, which were often used by hackers to trick victims into revealing their online passwords.

The company recently detected four fake Macron-themed domains being set up on digital infrastructure used by a group it called Pawn Storm, according to Feike Hacquebord, a Trend Micro researcher.

Mahjoubi confirmed that at least one of the sites had recently been used as part of an attempt to steal campaign staffers’ online credentials.

Unmasking groups behind any spying campaigns is one of the most challenging aspects of cybersecurity, but Hacquebord said he was confident that Trend Micro had succeeded. “This is not a 100% confirmation, but it’s very, very, likely,” he said, adding that the political nature of the targeting was “really in line with what they’ve been doing in the last two years”.

Trend Micro did not accuse any country of pulling the strings of Pawn Storm, a cyber espionage group. But US spy agencies and a variety of “threat intelligence” firms said that Pawn Storm, an extraordinarily prolific group also known as Fancy Bear or APT 28, was an arm of Russia’s intelligence apparatus.

French officials have tended to be more circumspect than their American counterparts, repeatedly declining to tie Pawn Storm to any specific source.

Russian government officials have long denied claims of state-sanctioned hacking. On Tuesday, Vladimir Putin’s spokesman, Dmitry Peskov, dismissed the most recent coverage as “anonymous, unsubstantiated reports”.

The Associated Press left several messages with the hacker or hackers who had registered the rogue Macron websites. No message was received in return.

Mahjoubi said the attempts to penetrate the Macron campaign dated to December 2016. In February this year, the campaign complained publicly of being targeted by Russia-linked electronic spying operations, although it offered no proof at the time.

Trend Micro’s report, which was produced independently of the Macron campaign and lists 160 electronic espionage operations across a series of targets, adds a measure of evidence to the notion – even if the fact that the rogue websites were registered in March and April did not line up with the campaign’s timeline.

The French election has been closely watched for signs of digital interference of any kind. Many observers fear a repeat of the US electoral contest in 2016, when hackers allegedly backed by Moscow broke into the email inboxes of the Democratic National Committee and other political operatives. Pilfered documents subsequently appeared on WikiLeaks and other more mysterious websites, putting the Democrats on the defensive during their losing campaign against Donald Trump, who became US president.