WASHINGTON -- Florida's Democratic senator said Wednesday that Russian operatives have penetrated some of his state's election systems ahead of this year's midterms, but state officials said they have no information to support the claim.

"They have already penetrated certain counties in the state and they now have free rein to move about," Nelson told the Tampa Bay Times.

Nelson, who is running for re-election, declined to identify which counties have been penetrated, saying it was classified. "The threat is real and elections officials -- at all levels -- need to address the vulnerabilities."

Nelson, the ranking member of the cyber subcommittee of the Senate Armed Services Committee, and his Republican colleague, Florida Sen. Marco Rubio, who is on the Senate intelligence committee, wrote a letter last month to all 67 of the county election supervisors in their state.

"We were requested by the chairman and vice chairman of our intelligence committee to let the supervisors of election in Florida that the Russians are in their records," Nelson said, adding that the letter also urged the county officials to seek help from the Homeland Security Department.

"This is no-fooling time. That's why two senators -- bipartisan -- reached out to the apparatus in Florida to let them know that the Russians are in the records and all they (the Russians) have to do -- if those election records are not protected -- is to go in and start eliminating registered voters.

"You can imagine the chaos that would occur on Election Day when the voters get to the polls and they say: 'I'm sorry, Mr. Smith. I'm sorry, Mr. Jones, you're not registered.' That's exactly what the Russians want to do."

The Florida Department of State said it had received "zero information" from Nelson or his staff that supports the claims of Russian meddling -- something national intelligence and homeland security officials have repeatedly warned was likely ahead of the midterms.

"Additionally, the department has received no information from the U.S. Department of Homeland Security, the Federal Bureau of Investigation or the Florida Department of Law Enforcement that corroborates Sen. Nelson's statement and we have no evidence to support these claims," the Florida Department of State said in a statement. "If Sen. Nelson has specific information about threats to our elections, he should share it with election officials in Florida."

The department said state and local election officials have taken "significant steps to ensure the security and integrity" of Florida elections.

Counties are using $1.9 million to purchase a network monitoring security system that provides automated alerts about threats, the department said. Moreover, counties are using $15.5 million in funding to make significant investments in election security prior to this year's midterms.

Sara Sendek, a spokesperson for the Department of Homeland Security, issued a statement Wednesday evening saying: "While we are aware of Senator Nelson's recent statements, we have not seen any new compromises by Russian actors of election infrastructure. That said, we don't need to wait for a specific threat to be ready."

The FBI did not immediately respond to requests for comment.

Rubio and Sen. Richard Burr, R-N.C., the chairman of the Senate intelligence committee, declined to comment.

The ranking Democrat, Sen. Mark Warner of Virginia, issued a statement, but did not confirm or deny Nelson's claims.

"Russian activities continue to pose a threat to the security of our elections, as Sens. Nelson and Rubio rightly pointed out in their letter," Warner said. "I hope all state and local elections officials, including Florida's, will take this issue seriously."

White House critics have harshly criticized President Donald Trump on the issue of election security. They say his administration lacks a clear national strategy to protect U.S. elections from foreign meddling by Russia or any other adversary. In response, top national security officials appeared together at the White House last week to insist there is a "vast, government-wide effort" to safeguard a cornerstone of American democracy.

John Bolton, the national security adviser, wrote in a letter to Senate Democrats that "President Trump has not and will not tolerate interference in America's system of representative government."

Director of National Intelligence Dan Coats said, "We continue to see a pervasive messaging campaign by Russia to try to weaken and divide the United States."

Their rare appearance during a White House briefing came weeks after Trump publicly undermined the conclusions of American intelligence agencies regarding Russian interference. After suffering a bipartisan outcry, Trump later said he accepted those findings.

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Associated Press Writers Lisa Mascaro in Washington, Frank Bajak in Boston and David Fischer in Miami contributed to this report.