Hackers stole classified information on dangerous chemicals in their raid on the South Korean army computer network in what was believed to be an attack by North Korea, Yonhap News Agency reported Saturday, quoting government officials.

The Chemicals Accident Response Information System, used by 589 South Korean government agencies including fire and police stations, was accessed by hackers on March 5. Hackers appear to have broken into the system using the ID of a South Korean army officer whose personal computer was infected by a virus, according to officials.

"We believe the hacker tapped into the system using the ID, withdrawing classified information of 1,350 dangerous chemicals," an army official was quoted as saying by Yonhap. "The government believes North Korea could be behind the hacking. We are continuing our investigation."

The revelation comes less than three months after cyber attacks severely slowed or disrupted dozens of South Korean government and business Web sites, including those of the presidential office and ministries of defense and foreign affairs. Rumors were rampant then that North Korean hackers orchestrated the attacks, although they have yet to be substantiated.

"We are trying to find out exactly how much information has been withdrawn," another unnamed official at Seoul's Environment Ministry was quoted as saying, adding the government has asked organizations dealing with the chemicals to tighten security.

Unnamed intelligence sources in Seoul said in May that North Korea operates a cyber warfare unit that seeks to disrupt South Korean and U.S. military networks and visits U.S. military sites more frequently than any other country.

South Korea and the U.S. signed a memorandum of understanding on April 30 to bolster cooperation in fighting cyber terrorism against their defense networks.