WASHINGTON - Russian hackers targeted 21 US states’ election systems in last year’s presidential race, a US Department of Homeland Security official told Congress on Wednesday.

Jeanette Manfra, the department’s acting deputy undersecretary of cyber security, would not identify which states had been targeted, citing confidentiality agreements. She reiterated to the Senate Intelligence Committee that there was no evidence that any actual vote ballots were manipulated.

Department officials had previously said about 20 states had been probed by hackers working on behalf of the Russian government, but recent media reports had suggested the number could have been far higher. Mark Warner, the top Democrat on the Senate panel, expressed frustration at Manfra’s refusal to identify which states had been targeted. “I just fundamentally disagree,” he said.

Arizona and Illinois last year confirmed that hackers had targeted their voter registration systems.

Russia has repeatedly denied responsibility for any cyber attacks during the US presidential election.