WASHINGTON (Reuters) - The top Democrat on the U.S. Senate Intelligence Committee said on Wednesday the panel had asked election officials in 21 U.S. states to make public information about efforts to hack their systems during last year's election.


Some lawmakers have expressed frustration that the information has been kept secret, saying it impedes the country's ability to prevent such hacks in the future.


"This week the Chairman (Republican Senator Richard Burr) and I sent a letter to all relevant state election officials asking that this information be made public," Senator Mark Warner said at an intelligence committee hearing.


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


A Homeland Security Department official testified to the intelligence committee on June 21 that Russian hackers targeted 21 U.S. state election systems in the 2016 presidential race and a small number were breached. [L1N1JI1R9]


Warner said he has asked the Department of Homeland Security to share the names of the 21 states, which have not been publicly identified.

The committee held a hearing on Wednesday on Russian efforts to influence elections in Europe, and European nations' responses.

U.S. intelligence agencies have concluded that Russian President Vladimir Putin's government was behind efforts to meddle in the U.S. election to boost Republican President Donald Trump's chances of winning the White House.

Russia has denied such assertions and Trump has dismissed them as sour grapes by Democrats disappointed about his surprise victory in November.

(Reporting by Patricia Zengerle and Dustin Volz; Editing by Dan Grebler)