The official's comment in a briefing on alleged foreign interference in the November presidential election comes amid a broad U.S. government push to counter such efforts.
On Wednesday, the U.S. charged two RT employees with money laundering, accusing them of participating in a scheme to have an American firm produce online content to influence the vote.
Those government actions were a further sign that Russia was using private Russian firms and RT "to covertly amplify and stoke domestic divisions and push for Russia's preferred electoral outcomes," the official said, speaking on condition of anonymity.
"RT has built and used networks of U.S. and other Western personalities to create and disseminate Russia-friendly narratives," the official said. "These actors among others are supporting Moscow's efforts to influence voter preferences in favor of the former president (Trump) and diminish the prospects of the vice president (Harris)."
The official did not provide further details of how those actors are trying to sway voters.
RT responded to the charges on Wednesday with ridicule, telling Reuters: "Three things are certain in life: death, taxes and RT's interference in the U.S. elections."
Russia previously has denied interfering in U.S. elections.
Russia is the most active foreign adversary trying to influence the election, while China is more focused on influencing local-level races, the official said.
U.S. officials have previously said they do not believe China was seeking to influence the outcome of the U.S. presidential election.
Chinese embassy spokesperson Liu Pengyu denied that China was interfering in U.S. election campaigns, calling the allegation "malicious speculation" and adding that Beijing "has no intention" of such interference.
The official said Iran had been more active than in past cycles, stepping up its efforts to sway voters leading up to the presidential and congressional election.
Iran's mission to the United Nations emailed an Iranian Foreign Ministry statement that rejected the allegation of Tehran's interference, calling it "baseless and biased for domestic political consumption."
The U.S. intelligence community is increasing the use of private warnings to targets of foreign influence operations, the official added.
Justice Department officials on Wednesday said the two RT employees used shell companies and fake personas to pay $10 million to an unidentified Tennessee company to produce online videos aimed at amplifying political divisions in the United States.
U.S. officials told a Senate committee in May that an increasing number of foreign actors, including non-state actors, are seeking to influence U.S. elections, and Russia, China and Iran, while the most significant, are far from alone.
