WASHINGTON — Sensitive information on spies reportedly has been found on at least one email on Hillary Clinton’s private server — as Defense Secretary Robert Gates warned it’s likely that several countries were able to hack into the server.

Fox News Channel, quoting two sources, said an intelligence agency had labeled the contents of the email dealing with one or more spies “HCS-O’’ — the code for reports on human intelligence sources in ongoing operations.

The report quotes a December 2013 policy document that says the designation “is used to protect exceptionally fragile and unique IC [intelligence community] clandestine [human intelligence] operations and methods that are not intended for dissemination outside of the originating agency.”

It’s not known if the information in the e-mails revealed the identity of the source.

The impact on the source, if any, as well as the findings of a damage assessment by the agency that ran the source, was not disclosed.

Meanwhile, Gates said China, Russia and Iran were among the countries with the ability to snoop on Clinton’s server.

“Given the fact that the Pentagon acknowledges that they get attacked [online] about 100,000 times a day, I think the odds are pretty high,” Gates told the Hugh Hewitt radio show Thursday night.

Asked if classified information was compromised, Gates said: “It would depend entirely on what she put on there.”

Congressional investigators reported in October that Clinton’s e-mail account was the target of hackers from China, South Korea and Germany. Russian hackers also tried to breach the account in 2011, hoping Clinton would click on e-mails sent as phony traffic tickets.

Clinton on Thursday night denied a report on Infowars.com that she continued using the private account even after being told it had been hacked. “It’s ­totally untrue,” she said.

The final batch of the controversial e-mails had been scheduled for release by Jan. 29 — two days before the Iowa caucuses.

But the blizzard that socked the East Coast was good news for Clinton.

On Friday, the State Department asked a federal judge to stretch the deadline to Feb. 29, citing the storm as a reason it can’t deliver the documents on time. State said it suddenly realized that 7,200 pages of e-mails haven’t been sent to other agencies for final review and that task has been made much more difficult by the snow, which sent federal workers home early.