Seven of former U.S. Secretary of State Hillary Clinton's private email chains will be withheld from public release because they have been found to contain top-secret information, the U.S. State Department said Friday.

"The State Department will be denying in full seven email chains, found in 22 documents representing 37 pages," State Department spokesman John Kirby said. "The documents are being upgraded at the request of the intelligence community because they contain a category of top-secret information ... These documents were not marked classified at the time they were sent."

The controversy over Clinton's use of a private email server in her New York home for her work as a secretary of state from 2009 to 2013 has dogged her campaign to become president.

With three days to go before the Iowa caucuses, when the first votes are cast for the Democratic and Republican presidential nominees, the announcement is likely to reignite criticism by her Republican opponents that she made highly sensitive government secrets vulnerable to hackers.

The government forbids the handling of classified information outside secure government-controlled channels.

The Clinton campaign downplayed the announcement as an example of "bureaucratic infighting" and "overclassification run amok" in a statement Friday.

"We firmly oppose the complete blocking of the release of these emails," the statement said.