US intelligence had evidence that voter registration systems or websites in seven states - Alaska, Arizona, California, Florida, Illinois, Texas and Wisconsin - were compromised by Russian-backed operatives before the 2016 election but never told the states, NBC News reported on Tuesday.
The Department of Homeland Security denied the report, a spokesman calling it "factually inaccurate and misleading" in a statement.
NBC, citing unnamed US officials, said that top-secret intelligence requested by former president Barack Obama in his last weeks in office synthesized months of work and made the conclusions.
Only Arizona and Illinois had previously been identified as states that experienced some level of intrusion into their election systems.
US intelligence agencies concluded in 2016 that Russia ran a program of hacking and disinformation to interfere in the elections and it later developed into an attempt to help Republican candidate Donald Trump defeat Democrat Hillary Clinton. On February 16, a US special counsel indicted 13 Russians and three companies, including St-Petersburg-based Internet Research Agency known for its trolling on social media, with charges of tampering in the campaign.
While officials in Washington told several of the states ahead of the elections that foreign entities were probing their systems, none were told the Russian government was behind it, NBC News reported, citing unnamed state officials.
The Trump administration contacted election officials in all 50 states in September 2017 to advise them whether or not their systems had been targeted. It told them 21 states had been targeted and some had been breached, NBC News reported.
Six of the seven states that were breached continued to deny it, citing their own cyber investigations, NBC News reported. It said the systems in the seven states were compromised in a variety of ways, including entry into state websites and penetration of voter registration databases.
All of the state and federal officials agreed that no votes were changed and no voters were removed from voter roles as a result of the activity by Russian operatives, NBC News reported.
The NBC report "is not accurate and is actively undermining efforts of the Department of Homeland Security to work in close partnership with state and local governments to protect the nation’s election systems from foreign actors," DHS spokesman Tyler Houlton said in the statement.
Houlton said formerly classified documents shown on the program were based on preliminary information and not validated intelligence on Russian activities.
Houlton said that the DHS is aware of 21 states being targeted by Russian hackers during the 2016 election cycle, and that in nearly all of them "only preparatory activity like scanning was observed."
"In no case is there any evidence that votes were changed or that Russian actors gained access to systems involved in vote tallying," he said.