A pro-Vladimir Putin hacking cell has announced plans to wreak havoc across official US websites over the next three days.
A tweet showing screenshots of the planned action, which is entitled "USA OFFLINE: F**K NATO", lists some of the targets, including state and federal websites.
For each of the websites, the hackers have detailed how they plan to disrupt the services.
They intend to block the main website for several of the authorities, while also "breaking all online services".
The plans were shared on Telegram by a pro-Kremlin hacking collective called Killnet, which has already targeted the online infrastructure of several Western governments this year.
Speaking about the group in September, hacktivism expert Stefan Soesanto told Politico: “Many in Russia see them as a hero."
"Killnet's aim is to make Europeans pay for their unequivocal support of Ukraine and punish Western governments for their anti-Russian sentiment," Soesanto, a researcher at the Center for Security Studies at ETH Zurich, added.
Their most high-profile attack was in May during the Eurovision song contest after Russia was banned from competing due to Putin's illegal war in Ukraine.
In response to the ban, the hacker group tried a DDoS attack, which was thwarted by Italian authorities - but they did suffer retaliation as a result.
It comes after MI5's website was down for part of Friday after a possible cyber attack.
The security service's public site was briefly unavailable for intermittent periods in the morning but is now back online - with the incident resolved.
The cause of the problem is being looked at but was considered to be a minor outage, the PA news agency understands.
The website is believed to have been subject to a distributed denial-of-service (DDoS) attack, which seeks to disrupt a site by flooding it with web traffic in a bid to try and knock it offline.
It is understood no sensitive information was held on, or connected to, the website and no data was lost.
Anonymous Russia, reportedly a group of pro-Russian hackers, has apparently claimed responsibility for the attack, but whether they were behind it has not been verified.
DDoS is a common form of cyber attack used by a wide range of perpetrators, the nature of which makes it difficult to attribute responsibility to a particular group.
A security source said the website was "intermittently unavailable this morning and is now back online", adding: "As is standard when this happens, the information members of the public might need to report anything suspicious was clearly displayed instead of the usual homepage."
