Since July 4th, major websites in the US and South Korea have been under attack by “a hostile group or government.”

SEOUL, South Korea — Cyberattacks that have crippled the Web sites of several major American and South Korean government agencies since the July 4th holiday weekend appear to have been launched by a hostile group or government, South Korea’s main government spy agency said on Wednesday.

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

A spokesman at the intelligence agency said it could not confirm the Yonhap report, which said that the spy agency briefed lawmakers about their suspicions on Wednesday. The opposition Democratic Party accused the spy agency of spreading unsubstantiated rumors to whip up support for a new anti-terrorism bill that would give it more power.

Access to at least 11 major Web sites in South Korea — including those of the presidential Blue House, the Defense Ministry, the National Assembly, Shinhan Bank, the 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.

On Wednesday, some of the sites regained service, but others remained unstable or inaccessible.

In an attack linked with the one in South Korea, 14 major Web sites in the United States — including those of the White House, the State Department and the New York Stock Exchange — came under similar attacks, according to anti-cyberterrorism police officers in Seoul.