Russian President Vladimir Putin likely authorised attempts to influence last year's US election in favour of former President Donald Trump, intelligence officials say.
Moscow spread "misleading or unsubstantiated allegations" about the eventual winner, Joe Biden, according to a US government report.
But it said no foreign government had compromised the final results.
Russia has repeatedly denied allegations of election interference.
The 15-page report, released on Tuesday by the Office of the Director of National Intelligence, outlined what it said were "influence operations" pushed by Russia as well as Iran.
It said Russian-linked individuals had spread unsubstantiated claims about President Biden ahead of the 3 November election. It also said a disinformation campaign sought to undermine confidence in the broader election process.
Some people connected to Russian intelligence also pushed anti-Biden narratives to media outlets, senior officials and allies of Mr Trump, the report said.
President Biden defeated Mr Trump and was inaugurated on 20 January.
The report added that, while Russia had sought to boost Mr Trump's chances of victory, Iran had launched a "multi-pronged covert influence campaign" in an effort to weaken his support.
The former president pursued a policy of "maximum pressure" on Iran, imposing damaging sanctions and escalating a war of words between the two nations.
The report also concluded with "high confidence" that China, which has long been accused of cyber-espionage by Washington, chose not to "deploy interference efforts" ahead of the vote.
"China sought stability in its relationship with the United States and did not view either election outcome as being advantageous enough for China to risk blowback if caught," it said.
According to the report, the voting process and final results were not interfered with by foreign states.
The intelligence report was released at the same time as a joint investigation by the departments of Justice and Homeland Security which came to a similar conclusion.
Their report said "broad Russian and Iranian campaigns targeting multiple critical infrastructure sectors did compromise the security of several networks that managed some election functions".
But it emphasised that the alleged interference attempts were largely indirect.
"We have no indications that any foreign actor attempted to interfere... by altering any technical aspect of the voting process, including voter registration, ballot casting, vote tabulation, or reporting results," the document reads.
The US intelligence community said last August that China, Russia and Iran were actively trying to meddle in the forthcoming presidential election.
The assessment found that Russia was seeking to "denigrate" Mr Biden. It also found that China and Iran wanted Mr Trump to lose the vote.