Russians penetrated multiple states' voter registration rolls in the 2016 presidential election, the head of cybersecurity at the Department of Homeland Security told NBC News on Wednesday.

Of the 21 states targeted by Russians in the election, several were successfully penetrated, said Jeanette Manfra, in an exclusive interview with NBC.

U.S. officials have said there is no evidence that any states' voter registration rolls were tampered with or at all altered by the foreign intrusions.

Manfra, whose role is to protect American elections from hacking, did not specify which states had been successfully penetrated, saying she could not publicly disclose classified information.

"We saw a targeting of 21 states and an exceptionally small number of them were actually successfully penetrated," Manfra told NBC.