South Korea says it fears it has come under cyber attack from North Korea in the wake of the nuclear test that alarmed the world.
The country's unification ministry confirmed an attack had taken place but refused to give details of what targets had been hit.
Jeong Joon-hee, a spokesman for the ministry, told a news conference: "At this point, we suspect it is an act by North Korea."
Mr Jeong said the authorities were investigating.
South Korea has been on heightened alert since 6 January, when Pyongyang claimed to have successfully tested a hydrogen bomb.
Seismologists recorded a jolt to the earth's crust similar to an earthquake at a point where the North's nuclear tests had previously taken place, but analysts said it was more likely to be a blast involving a material other than hydrogen.
Since then, South Korean President Park Geun-hye has said North Korea has been expanding the range of threats to include cyber warfare and the use of drones across its borders.
North Korea has also been using balloons to drop propaganda leaflets in the South.
In recent days, there have been unconfirmed news reports that the computer systems of some South Korean government agencies and companies have been infected with malicious codes that might have been sent by the North.
Some Korean media reported that attempts had been made to infect firms like Samsung with a form of malware similar to that used in a cyber attack on Sony Pictures in 2014.
Defectors from the North have said the North's spy agency operates a sophisticated cyber-warfare unit that attempts to hack and sabotage enemy targets.
South Korea and the United States blamed North Korea for the Sony Pictures attack that caused huge embarrassment to the firm and several of its top employees and film stars.
At the time, the company was set to release the film The Interview, which was a fictional plot to assassinate North Korean leader Kim Jong-un.
North Korea denied being responsible.
http://news.sky.com/story/1630531/north-korea-carries-out-cyber-attack-on-south