Microsoft said that a hacking group tied to the Russian government created fake internet domains that appeared to spoof two American conservative organizations: the Hudson Institute and the International Republican Institute. Three other fake domains were designed to look as if they belonged to the U.S. Senate.
Microsoft didn't offer any further description of the fake sites.
The Russian authorities denied allegations, the Interfax news agency reported on Tuesday. Interfax cited an unnamed Russian diplomatic source on Tuesday as describing Microsoft's allegations as part of a political game.
"Microsoft is playing political games," it cited the source as saying. "The (mid-term U.S.) elections have not happened yet, but there are already allegations."
Microsoft was acting like a prosecutor rather than a private company, Interfax quoted the source as adding.
The revelation came just weeks after a similar Microsoft discovery led Sen. Claire McCaskill, a Missouri Democrat who is running for re-election, to reveal that Russian hackers tried unsuccessfully to infiltrate her Senate computer network.
The hacking attempts mirror similar Russian attacks ahead of the 2016 election, which U.S. intelligence officials have said were focused on helping to elect Republican Donald Trump to the presidency by hurting his Democratic opponent, Hillary Clinton.
This time, more than helping one political party over another, "this activity is most fundamentally focused on disrupting democracy," Brad Smith, Microsoft's president and chief legal officer, said in an interview this week.
Smith said there is no sign the hackers were successful in persuading anyone to click on the fake websites, which could have exposed a target victim to computer infiltration, hidden surveillance and data theft. Both conservative think tanks said they have tried to be vigilant about "spear-phishing" email attacks because their global pro-democracy work has frequently drawn the ire of authoritarian governments.
"We're glad that our work is attracting the attention of bad actors," said Hudson Institute spokesman David Tell. "It means we're having an effect, presumably."
The International Republican Institute is led by a board that includes six Republican senators, and one prominent Russia critic and Senate hopeful, Mitt Romney, who is running for a Utah seat this fall.
Microsoft calls the hacking group Strontium; others call it Fancy Bear or APT28. An indictment from U.S. special counsel Robert Mueller has tied it to Russian's main intelligence agency, known as the GRU, and to the 2016 email hacking of both the Democratic National Committee and the Clinton campaign.
"We have no doubt in our minds" who is responsible, Smith said.
Microsoft has waged a legal battle with Strontium since suing it in a Virginia federal court in summer 2016. The company obtained court approval last year allowing it to seize certain fake domains created by the group. It has so far used the courts to shut down 84 fake websites created by the group, including the most recent six announced Tuesday.
Microsoft has argued in court that by setting up fake but realistic-looking domains, the hackers were misusing Microsoft trademarks and services to hack into targeted computer networks, install malware and steal sensitive emails and other data.
Smith also announced Tuesday that the company is offering free cybersecurity protection to all U.S. political candidates, campaigns and other political organizations, at least so long as they're already using Microsoft's Office 365 productivity software. Facebook and Alphabet's Google have also promoted similar tools to combat campaign interference.
—Reuters contributed to this report.