Russian hackers targeted election systems in 21 states in last year's United States presidential race, a Department of Homeland Security official has told Congress.
But a former Obama official added that a video of US President Donald Trump bragging of sexual conquests distracted attention from Washington's warnings.
- Official won't identify specific states
- Senators push for more details about the hacking
- Arizona and Illinois last year confirmed they were targeted
Jeanette Manfra, the department's acting deputy undersecretary of cyber security, would not identify which states had been targeted, citing confidentiality agreements.
She reiterated that there was no evidence that any actual votes were manipulated.
"As of right now, we have evidence that election-related systems in 21 states were targeted," Ms Manfra told the Senate Intelligence Committee, which is investigating Russia's meddling in the 2016 presidential election.
Systems were breached in a smaller number of states, she said, but did not give a specific figure.
Jeh Johnson, who led the Homeland Security Department until the end of the Obama administration, added that his department had issued warnings about hacking into voter registration databases.
But he told the US House Intelligence Committee, which is investigating alleged Russian meddling in the election, that the notices did not get the attention he would have liked, blaming the emergence of a 2005 tape — in which Mr Trump brags about sexually assaulting women — for distracting the American public.
The extent of interference by Russian hackers has been the source of speculation and media reports for months — the Russia issue has cast a shadow over Mr Trump's first five months in office.
Department officials had said about 20 states had been probed by hackers working on behalf of the Russian Government, but recent news media reports had suggested the number could have been far higher.
Mark Warner, the top Democrat on the Senate panel, expressed frustration at Ms Manfra's refusal to identify which states had been targeted.
"I just fundamentally disagree," he said.
Senator Warner on Tuesday sent a letter to Homeland Security Secretary John Kelly asking the agency to disclose more information about hacking attempts on state and local election systems.
Small amount of votes could swing election: senator
Arizona and Illinois have confirmed that hackers had targeted their voter registration systems.
US intelligence agencies concluded that the Kremlin orchestrated a wide-ranging influence operation that included email hacking and online propaganda in order to discredit Democratic presidential candidate Hillary Clinton and help Mr Trump win the White House.
Ms Manfra and other officials testifying on Wednesday reiterated that US elections are resilient to hacking in part because they are decentralised and largely operated on the state and local level.
Senator Angus King, an independent from Maine who caucuses with the Democrats, expressed scepticism at that assertion, saying only a small amount of votes in key battleground states would need to be altered to tip the scales of an election.
"A sophisticated actor could hack an election simply by focusing on certain counties," Senator King said.
"I don't think it works just to say it's a big system and diversity will protect us."
Russia has repeatedly denied responsibility for any cyber attacks during the US presidential election.
Mr Trump has inconsistently said Russia may or may not have been responsible for the hacking, but has dismissed allegations that his associates colluded with Moscow as "fake news".