(Reuters) - The U.S. Department of Health and Human Services, a key part of the federal response to the fast-spreading coronavirus outbreak, was hit by an unspecified “cyber incident” on Sunday, officials said on Monday.

HHS Secretary Alex Azar said there was “no data breach.” National Security Council spokesman John Ullyot said HHS networks “are functioning normally at this time” and that officials are investigating the matter.

Department spokeswoman Caitlin Oakley said there was a “significant increase in activity on HHS cyber infrastructure” on Sunday but that HHS was “fully operational.”

Neither Ullyot nor Oakley gave other details as to the nature of the problem. Bloomberg News, citing unnamed sources, said there had been multiple hacking incidents that appeared aimed at slowing the department’s systems.

On Twitter, a Bloomberg reporter said the incident involved “overloading the HHS servers with millions of hits” - an apparent reference to a denial-of-service in which a fire hose of digital traffic is directed toward a website in a bid to knock it offline.

Such incidents are common and rarely bring down government sites. The department website appeared to be accessible on Monday.

Bloomberg tied the incident to the release of a statement by the National Security Council just before midnight on Sunday that denied rumors of a national quarantine, saying that text messages suggesting otherwise were fake. “There is no national lockdown,” said the statement, which was posted to Twitter.

It is not immediately clear how the denial of service would have been related to the fake quarantine rumors.

Reacting to the report, Senator Ben Sasse of Nebraska said that Americans “should expect an increase in cyberattacks and stay vigilant” as the nation increasingly becomes absorbed in the fight against the virus.