Several important US government web sites were shut down by hackers over the 4 July holiday weekend.
The Treasury Department, Secret Service, Federal Trade Commission and Transportation Department sites were all shut down for as long as two days after unknown parties carried out denial-of-service (DoS) attacks.
Amy Kudwa, a spokeswoman for the Homeland Security Department, told Associated Press that the department's US Computer Emergency Readiness Team issued a notice to federal departments and other partner organisations about the problems and "advised them of steps to take to help mitigate against such attacks".
The attacks have continued for more than three days, which is unusual for DoS attacks which are usually short in duration.
Problems are still continuing on some sites, although most web pages are now up and running. The identity of the group behind the attack is not known, but worryingly similar attacks took place over the weekend in South Korea.
South Korean government agency sites, banks and internet sites were hit, and Ahn Jeong-eun, a spokeswoman at the Korea Information Security Agency, said that the US and South Korean incidents appeared to be linked.
The attacks can only heighten tension with North Korea, after the missile tests conducted by the communist regime over the past weeks.
DoS attacks are increasingly used as a form of protest and disruption to mirror real-world conflicts. The first large scale example of this came in Estonia, but similar cases have been seen in Georgia and elsewhere.
Cotton seedling freezes to death as Chang'e-4 shuts down for the Moon's 14-day lunar night
Fortnite easily out-earns PUBG, Assassin's Creed Odyssey and Red Dead Redemption 2 in 2018
Meteor showers as a service will be visible for about 100 kilometres in all directions
Saturn's rings only formed in the past 100 million years, suggests analysis of Cassini space probe data
New findings contradict conventional belief that Saturn's rings were formed along with the planet about 4.5 billion years ago