Defence ministry also claims improved missile technology may have given Pyongyang ability to strike US mainland

South Korea has claimed that North Korea has a 6,000-member cyber-army dedicated to disrupting its military and government.

The figure is a doubling of its earlier estimate that the North had a cyberwarfare staff of 3,000.

Seoul’s defence ministry said in a report that Pyongyang may also have gained the ability to strike the US mainland because of its recent progress in missile technology, which was demonstrated in five long-range missile tests in 2009 and 2012, and is advancing in efforts to miniaturise nuclear warheads to mount on such missiles.

The US accused North Korea of a cyber-attack on Sony Pictures over a movie depicting the fictional assassination of the North’s leader, Kim Jong-un. North Korea has denied any involvement in the hacking of tens of thousands of confidential Sony emails and business files.

Former South Korean defence minister Kim Kwan-jin said in 2013 that North Korea was operating a cyberwarfare staff of 3,000. The South has accused the North of conducting at least six high-profile cyber-attacks since 2007 and many more unsuccessful attempts to infiltrate the computer systems of businesses and government agencies.

The Korean peninsula is still in a technical state of war because the 1950-53 Korean war ended in an armistice, not a peace treaty.