Former US Secretary of State Hillary Clinton. REUTERS/Scott Morgan The Obama administration is confirming for the first time that Hillary Clinton's unsecured home server contained some closely guarded secrets, including material requiring one of the highest levels of classification.

The revelation comes just three days before the Iowa presidential nominating caucuses in which Clinton is a candidate.

The State Department will release more emails from Clinton's time as secretary of state later Friday.

But The Associated Press has learned that seven email chains — reportedly containing 22 emails — are being withheld in full for containing "top secret" material.

Department officials wouldn't describe the substance of the emails, or say if Clinton had sent any herself.

Spokesman John Kirby told the AP that no judgment on past classification was made. But the department is looking into that, too.

Brian Fallon, the Clinton campaign's press secretary, called the State Department's declaration "overclassification run amok."

"We firmly oppose the complete blocking of the release of these emails. Since first providing her emails to the State Department more than one year ago, Hillary Clinton has urged that they be made available to the public. We feel no differently today," he said in a statement.

Of the nature of the emails, he added: "We understand that these emails were likely originated on the State Department's unclassified system before they were ever shared with Secretary Clinton, and they have remained on the department's unclassified system for years. And, in at least one case, the emails appear to involve information from a published news article."

The 37 pages of withheld emails include messages recently described by a key intelligence official as concerning so-called special access programs — a highly restricted subset of classified material that could point to confidential sources or clandestine programs like drone strikes or government eavesdropping.

These emails have since been allocated a special-access program (SAP) designation, according to a letter that the intelligence community's inspector general, Charles McCullough, sent to lawmakers this month.

SAPs are designed to safeguard information deemed more sensitive than even "top secret."

Kirby also said that some emails between Clinton and President Barack Obama would be withheld. He said the department had discovered 18 emails between the secretary and president. None will be publicly released. Kirby said none of those emails contained classified information.

The FBI has also been looking into whether classified material was mishandled during Clinton's tenure at the State Department from 2009 to 2013. Clinton, the front-runner for the Democratic presidential nomination, has repeatedly insisted that she never sent or received information on her personal account that was marked classified at the time.

No emails released so far were stamped "CLASSIFIED" or "TOP SECRET," but reviewers previously had designated more than 1,000 messages at lower classification levels for public release. Friday's will be the first at the top secret level.

For those that Clinton only read, and didn't write or forward, she still would have been required to report classification slippages that she recognized. But without classification markings, that may have been difficult, especially if the information was in the public domain.

Kirby said the State Department's focus as part of the Freedom of Information Act review of Clinton's emails was on "whether they need to be classified today." Questions about their past classification, he said, "are being, and will be, handled separately by the State Department."

Possible responses for classification infractions include counseling, warnings or other action, State Department officials said, though they declined to say if these applied to Clinton or senior aides who've since left the department. The officials weren't authorized to speak on the matter and spoke on condition of anonymity.

Friday's release is coming at an awkward time for Clinton. The Iowa caucus is on Monday, and her main challenger, Sen. Bernie Sanders (I-Vermont), is running neck and neck with her in the polls there and leads solidly in New Hampshire. Clinton still holds a strong advantage in national polls.

The emails have been an issue for Clinton's campaign since it became known 10 months ago that she exclusively used a nongovernment account linked to a homebrew server while in office. Clinton first called the decision a matter of convenience and then termed it a mistake, even if doing so wasn't expressly forbidden. But the matter could prove more troublesome now that Clinton's former agency has confirmed that business conducted over the account included top-secret matters.

Clinton and Bernie Sanders. NBC/YouTube

Like Clinton, the State Department discounted such a possibility last March. Both also said her account was never hacked or compromised, which security experts assess as unlikely, and that the vast majority of her emails were preserved properly for archiving purposes because she corresponded mainly with government accounts. They've backtracked from the archiving claim, while the AP discovered several phishing attempts on her server connected to Russia.

The question of special access programs first surfaced last week, when McCullough, the inspector general for U.S. intelligence agencies, cited examples on Clinton's account in a letter to Congress. Republicans pounced on the report, though Clinton's campaign insisted none of the exchanges were "classified at the time" and accused McCullough and GOP lawmakers of selectively leaking materials to damage her presidential hopes.

Kirby confirmed that the "denied-in-full emails" are among those McCullough recently cited. One of the emails, he said, was among those McCullough identified last summer as possibly containing top secret information.

The AP reported last August that one focused on a forwarded news article about the classified U.S. drone program run by the CIA. Such operations are widely covered and discussed in the public sphere, including by top U.S. officials, and the State Department immediately pushed back against McCullough's claim. The other concerned North Korean nuclear weapons programs, according to officials.

At the time, several officials from different agencies suggested the disagreement over the drone emails reflected the government's tendency to over-classify material, and the lack of consistent policies across different agencies about what should and shouldn't be classified.

The FBI also is looking into Clinton's email setup, but has said nothing about the nature of its probe. Independent experts say it is highly unlikely that Clinton will be charged with wrongdoing, based on the limited details that have surfaced up to now and the lack of indications that she intended to break any laws.

"What I would hope comes out of all of this is a bit of humility" and an acknowledgment from Clinton that "I made some serious mistakes," said Bradley Moss, a Washington lawyer who regularly handles security clearance matters.

Legal questions aside, it's the potential political costs that are probably of more immediate concern for Clinton. She has struggled in surveys measuring her perceived trustworthiness and an active federal investigation, especially one buoyed by evidence that top secret material coursed through her account, could negate one of her main selling points for becoming commander in chief: Her national security résumé.

Natasha Bertrand contributed to this report.