US federal computer sites have been subjected to a fourth day of cyberattack, with the US State Department saying its website was under attack.

State Department spokesman Ian Kelly told reporters on Thursday that while there was not a high volume of attacks, they were ongoing and were causing the department “concern”.

“We are taking measures to deal with this and any potential new attacks,” Kelly added.

About a dozen US government websites, including those of the White House, the Pentagon, the Department of Homeland Security (DHS), and the State Department, were targeted in a coordinated cyberattack that began on July 4.

Simultaneously, government and commercial websites in South Korea were also targeted, including those of the presidential Blue House, the Defense Ministry, South Korea’s National Intelligence Service, major banks, and leading newspapers.

Despite speculation earlier in the week that North Korea was behind the attacks, known as a Distributed Denial of Service (DDoS), Kelly said, “I have no information… of North Korean involvement. I have… nothing that I can confirm”.

Kelly said that the department’s’ site based at the US embassy in Seoul, South Korea, was not shut down and was not materially affected by any of these attacks.

He said the US computer emergency readiness team is working with State Department experts, the DHS, and other government agencies to try to resolve the problem.

A denial of service attack attempts to paralyse a website by flooding it with traffic from an army of virus-infected computers known as a ‘botnet.’