South Korea's spy agency said Wednesday it had issued an alert against cyberattacks aimed at stealing data from government networks.

The National Intelligence Service (NIS) did not say whether North Korea was responsible.

Open Radio for North Korea, a Seoul-based group specialising in the North, said the latest attack was led by Pyongyang, which runs elite hacker units.

The NIS said its alert was heightened "from normal to concern" on Monday after a massive inflow of overseas hacking attacks.

The attacks were aimed at stealing data from government and other state networks, it said in a statement on its website.

The spy agency last year blamed the North for cyberattacks that briefly crippled unclassified US and South Korean government and commercial websites in July that year.

In 2004 hackers based in China used information-stealing viruses to break into the computer systems of Seoul government agencies.

Last December hackers using an Internet Protocol address in China gained access to parts of a secret US-South Korean plan to defend the peninsula in case of war.

South Korea, one of the world's most wired societies, this month launched a cyber warfare command centre manned by some 200 specialists.