The US Department of Health and Human Services has been hit by an unspecified "cyber incident", officials say.
National Security Council spokesman John Ullyot said department networks are now functioning normally. He said officials were investigating.
Department spokeswoman Caitlin Oakley said there was a "significant increase in activity on HHS cyber infrastructure" on Sunday but that the agency was "fully operational".
Neither Ullyot nor Oakley gave other details as to the nature of the problem, but Bloomberg News, citing unnamed sources, said there were 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, where a firehose 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 rumours 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.
Reacting to the report, Senator Ben Sasse of Nebraska said Americans should expect an increase in cyberattacks and stay vigilant as the nation increasingly becomes absorbed in the fight against the virus.