An image of Prince William, Princess Kate and the couple's three children
A note from the US Department of Health and Human Services referred to Killnet as a "hacktivist group".
It added: "KillNet is a pro-Russian hacktivist group active since at least January 2022 known for its DDoS campaigns against countries supporting Ukraine, especially NATO countries since the Russia-Ukraine war broke out last year.
"DDoS is the primary type of cyber-attack employed by the group which can cause thousands of connection requests and packets to be sent to the target server or website per minute, slowing down or even stopping vulnerable systems.
"While KillNet’s DDoS attacks usually do not cause major damage, they can cause service outages lasting several hours or even days.
Another stock hacker image
"Although KillNet’s ties to official Russian government organizations such as the Russian Federal Security Service (FSB) or the Russian Foreign Intelligence Service (SVR) are unconfirmed, the group should be considered a threat to government and critical infrastructure organizations including healthcare."
Killnet targeted members of the Royal Family in a separate cyber attack last November.
Prince William and Princess Kate were the pair impacted during last year's incident.
Killnet added: "Today it does not work, perhaps this is due to the supply of high-precision missiles to Ukraine!"
The Daily Telegraph separately reported that hackers in Russia and Iran might try to disrupt King Charles III's Coronation in May.
The Metropolitan Police foiled a plot to impact the Prince and Princess of Wales' wedding in 2011.