North Korea, usually blamed for hacking others, has accused the US of staging cyber attacks against its Internet servers after reports of disruptions to its main news services, the latest twist from an increasingly belligerent North.

Russia's Itar-Tass news agency said a "powerful hacker attack" from abroad had brought down internet servers inside the North, disabling access to some websites.

The accusation comes at a time of increased tension between North Korea and South Korea, along with the South's ally, the US.

South Korean and US military forces are taking part in exercises at a time of heightened tension in the Korean Peninsula.

North Korea claims the ongoing drills are invasion rehearsals. Pyongyang has threatened to reduce the South to a "sea of fire" and stage pre-emptive nuclear attacks on Washington in retaliation for new UN sanctions over its latest nuclear test.

South Korea vows that if it is attacked, it will respond with even greater force. The country's cyber television said the North's state media services were among those affected by the cyber attack.

Websites disrupted

These included the websites of the KCNA news agency and the official Rodong Sinmun newspaper, which were said to be experiencing disruptions even though they were operating normally on Thursday and Friday.

"It is nobody's secret that the US and the South Korean puppet regime are massively bolstering up cyber forces in a bid to intensify the subversive activities and sabotages against the DPRK," KCNA said on Friday.

The Democratic People's Republic of Korea, or DPRK, is the North's official name.

KCNA and Rodong Sinmun have carried the North's increasingly strident rhetoric of late, accusing the US and South Korea of staging preparations for war and vowing to scrap the armistice that stopped fighting in the 1950-53 Korean war.

The North has also threatened to use nuclear weapons against what it called hostile forces.

North Korea in turn has been blamed for spreading malicious software that crashed the websites of government agencies and businesses, and for a cyber attack on a South Korean state-run bank server in 2011 that took more than a week to restore.

North Korea denies charges of cyber attacks and accuses the South of a conspiracy to fuel confrontation, although defectors from the North have warned that Pyongyang was recruiting thousands of computer engineers to its cyber warfare unit.

Military experts said cyber warfare was a major threat from North Korea, along with its conventional forces and its weapons of mass destruction programme, that posed a security risk to utilities and communications networks in the South.