The Office of the Director of National Intelligence and the Department of Homeland Security said in a statement on October 7 that it is "confident" that the Russian government "directed" the hacking of e-mails of individuals and groups, including the Democratic National Committee (DNC), RFE/RL reported.
"We believe, based on the scope and sensitivity of these efforts, that only Russia's senior-most officials could have authorized these activities," the statement said.
Kremlin spokesman Dmitry Peskov dismissed the U.S. statement as "some kind of nonsense."
"Every day there are tens of thousands of attacks on Putin's website. Many of the attacks can be traced to the U.S.," Peskov was quoted as telling the Interfax news agency. "We're not blaming the White House or Langley every time," he added, referring to the Virginia city where the CIA is based.
Private cybersecurity firms have previously said hacking groups allegedly tied to Russian security services are behind the hacks, though until October 7, U.S. President Barack Obama's administration had refrained from publicly implicating the Russian government.
The compromised e-mails have been published by WikiLeaks and another website, DCLeaks.com. They prompted the resignation of the DNC chairwoman after internal communications showed staff members favoring Democratic presidential nominee Hillary Clinton over rival Bernie Sanders during the party's primaries.
Relations with Russia have become a central issue in the November 8 U.S. presidential election between Clinton and her Republican rival, Donald Trump, who has spoken admiringly of Russian President Vladimir Putin and said he would seek to improve ties with Moscow if elected.
Clinton's campaign team has repeatedly suggested the Russian government was behind cyberattacks that targeted the DNC, alleging that Moscow has been trying to help Trump win the vote.
Both Trump and the Kremlin have called the allegation absurd.
U.S. Secretary of State John Kerry said in July said that he had raised the issue with Russian Foreign Minister Sergei Lavrov, who brushed off a question about possible Russian involvement in the affair by saying, "I don't want to use four-letter words."
Read alsoFBI probes hacks targeting phones of Democratic Party officials – ReutersThe October 7 statement from Washington noted that several U.S. states have been subjected to "scanning and probing of their election-related systems," primarily from "servers operated by a Russian company." But it said intelligence officials "are not now in a position to attribute this activity to the Russian government."
Naming Russia as the actor behind the cyberattacks on political organizations falls short of more punitive measures the United States has taken against other countries for cyberintrusions, which have included sanctions and prosecutions against the hackers.
Republican Senator Cory Gardner, chairman of the U.S. Senate Foreign Relations Subcommittee on East Asia, the Pacific and International Cybersecurity, said he planned to introduce sanctions legislation over "Russia's cybercriminals."
The announcement comes amid a continuing deterioration of relations between Washington and Moscow, most notably over an ongoing Russian-backed Syrian government offensive on the city of Aleppo that has killed hundreds of civilians since a Russian and U.S.-brokered cease-fire deal broke down on September 19.