The Senate Intelligence Committee has asked election officials in 21 states to make public information about Russian efforts to hack their elections systems during the 2016 elections, the panel's top Democrat said Wednesday.

The request was made in a letter sent last week "to all relevant state election officials" from Sens. Richard Burr (R-N.C.) and Mark Warner (D-Va.), the panel's chairman and vice chairman, respectively, Warner revealed in his prepared remarks before a hearing on global election interference.

"I do not see how Americans are made safer when they do not know which state elections systems Russia tried to hack," Warner said.

Department of Homeland Security (DHS) officials last week revealed that Russian hackers targeted election-related systems in 21 states leading up to Election Day.

But officials would not disclose which states were targeted, citing the need to protect the confidentiality of the states affected. Some lawmakers have expressed frustration at the department's reticence, arguing that it makes it more difficult to prevent such attacks in the future.

"That is why I have asked DHS to share with this committee the names of the 21 states they testified last week were attacked by Russian hackers," Warner said, adding that he has written to and spoken with Homeland Security Secretary John Kelly on the matter.

"As the oversight committee for all intelligence issues, this committee is entitled to have this information."

The Obama administration had previously revealed that voter databases in Arizona and Illinois were breached by foreign-based hackers. It is unclear whether any of the remaining state systems were successfully breached.

"We need to work through a process so state election officials have the security clearance to at least be read in," Warner said, saying that the top official from neither Indiana nor Wisconsin could acknowledge whether they were among the 21.

Officials have said there is no doubt Russia was behind the attacks - but that the impacted systems were not involved in actual vote counting.