Cyberattacks that have crippled the websites of several major American and South Korean Government agencies since the July 4 appear to have been launched by a hostile group or Government, South Korea’s main spy agency said on Wednesday.

Although the National Intelligence Service did not identify whom they believed responsible, South Korean news agency Yonhap reported that the spy agency had implicated North Korea or pro-North groups.

A spokesman at the intelligence agency said it could not confirm the Yonhap report. The Opposition Democratic Party accused the spy agency of spreading unsubstantiated rumours to whip up support for a new anti-terrorism bill that would give it more power.

Access to 11 major websites in South Korea — including those of the presidential Blue House, Defence Ministry, National Assembly, Shinhan Bank, mass-circulation daily newspaper Chosun Ilbo and the top Internet portal Naver.com — have crashed or slowed down to a crawl since Tuesday evening, according to the Government’s Korea Information Security Agency.

Fourteen major websites in the United States — including those of the White House, the State Department and the New York Stock Exchange — came under similar attacks.