Hackers in China have stolen secrets on South Korea's defence and foreign affairs by using bogus emails claiming to come from Seoul officials and diplomats, the intelligence agency said Friday.

The National Intelligence Service uncovered the hacking early this year and warned government offices about the danger of such emails, a spokesman told AFP.

Hackers sent emails in the names of South Korean diplomats, presidential aides and other people familiar to Seoul officials.

Attached files containing viruses were disguised as important documents, such as analyses on North Korea's economy.

When a recipient clicked on the attachment, the virus started downloading documents in his or her computer, the spokesman said.

Lawmaker Lee Jung-Hyun of the ruling Grand National Party told parliament Thursday that a "considerable volume of classified documents" was feared to have been leaked from the defence ministry and the foreign ministry.

The foreign ministry said it had asked overseas diplomatic missions to be extra alert to such hacking attempts.

The South's intelligence service in June investigated a major "distributed denial of service" cyber attack on the main government website by hackers traced to China.

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

The South's spy chief blamed North Korea for cyber attacks from China-based servers that briefly crippled US and South Korean government and commercial websites in July 2009. US officials were uncertain of the origin.

Seoul military officials say the North has an army unit of elite hackers.