A conservative South Korean newspaper said Monday it had been the victim of a major cyber attack, less than a week after North Korea threatened the paper and other Seoul media over their reports.

Police are investigating Pyongyang's possible involvement in the hacking of the Internet news site and database server at the JoongAng Ilbo and sister paper the Korea JoongAng Daily, the Daily reported.

The English-language Daily said both papers lost the databases for articles and photos. The editing system that moves copy was also destroyed, disrupting production, by the cyber attack Saturday evening.

The paper, in a news report, said the connection to the site www.joongang.co.kr was cut off. A photo of a white cat and the statement "Hacked by IsOne" was posted instead.

It said a message suggested that more media sites would be hacked on two days, the 19th and the 29th, although the month was not specified.

North Korea's military on June 4 threatened attacks on the Seoul offices of South Korean media outlets including JoongAng Ilbo, for their critical coverage of a mass children's event in Pyongyang.

The military general staff listed the coordinates of some of the offices and said missile units and other forces had already targeted the buildings.

It did not mention a possible cyber-attack, although Seoul says its neighbour carried out several such attacks in the past.

"We have never seen a strong attack like this before," the paper quoted Jong Seok-Hwa, chief investigator of the national police force's Cyber Terror Response Center, as saying.

"We will find out the origin of the hacking and take necessary measures."

Unidentified officers were quoted as saying they are investigating possible North Korean involvement. Police and the newspaper could not immediately be reached for comment.

The Korea JoongAng Daily said the hacker connected to the website by typing in a domain address used only by employees.

It said the two papers are using a temporary server, and plan to normalise production as soon as possible by backing up lost data and installing a new operating system.

Seoul says Pyongyang has an elite team of hackers.

It accused the North of staging cyber-attacks on websites of South Korean government agencies and financial institutions in July 2009 and March 2011.

In May 2011 the South said a North Korean cyber attack paralysed operations at one of its largest banks. Pyongyang says Seoul invented the charges.

Last month Seoul accused Pyongyang of jamming the GPS systems of hundreds of civilian aircraft and ships in South Korea between April 28 and May 13. The North rejected the South's accusations as "sheer fabrication".