Prosecutors in South Korea say North Korean hackers were behind an attack that paralysed a leading bank last month. Banking operations at Nonghyup, a South Korean farm co-operative, were halted by the cyber intrusion, leaving customers unable to access their money.

The Seoul prosecutors' office called it "unprecedented cyber-terror deliberately planned" by North Korea.It said the software used matched that used in earlier attacks by Pyongyang.

Prosecutors said that a laptop used by a subcontractor "became in September 2010 a zombie PC operated by the North, which... later remotely staged the attack through the laptop".

One of the Internet Protocol (IP) addresses used to break into Nonghyup's system was the same as one used in March for a distributed denial-of-service (DDoS) attack that originated in North Korea, they added.

The software used in the incident was also similar to that employed in July 2009, when a number of South Korean government websites were attacked, the prosecutors said.

The latest attack caused a three-day service outage at the bank - also called the National Agricultural Co-operative Federation - and caused the records of some credit card customers to be deleted.

The two Koreas technically remain at war following the 1950-53 Korean War, and tensions have been high in recent months in the wake of two deadly incidents.