South Korea's intelligence service is investigating a major cyber attack on the main government website by hackers traced to China, officials said Thursday.

The attacks on Wednesday evening lasted around three-and-a-half hours, slowing traffic on the site (http://korea.go.kr) which provides information on policies and services, said the Ministry of Public Administration and Security.

The ministry said its cyber security team had been on alert for such attacks as tensions rose with North Korea.

Hackers used about 120 China-based Internet servers, a ministry spokesman told AFP, adding the government took immediate measures to thwart the "distributed denial of service" (DDoS) attacks.

"We are now tracing the origin of the attacks" in cooperation with intelligence authorities and other government offices, he said.

South Korea's spy chief blamed North Korea for cyber attacks from China-based servers that briefly crippled US and South Korean government and commercial websites last July.

South Korea announced reprisals against the North, including a trade cut-off, after investigators concluded last month that a North Korean submarine torpedoed a South Korean warship with the loss of 46 sailors in March.

The North furiously denies involvement and has responded to the reprisals with threats of war.

On Tuesday the head of South Korea's military intelligence unit warned that the North may follow up its ship attack with cyber attacks to disrupt the Group of 20 summit in Seoul in November.

Major General Bae Deuk-Shik, chief of the Defence Security Command, said the North has an army unit of elite hackers.

Intelligence officials have also said the North mounted a cyber campaign -- using stolen identities of South Korean Internet users -- to spread its own claims about the sinking.

DDos attacks are designed to swamp selected websites with traffic.