Similar jamming in the past was traced to the reclusive North, which last month breached U.S. Security Council resolutions with a failed long-range rocket launch and was blamed for cyber attacks on South Korean financial institutions last year.

None of the flights, including 11 operated by foreign airlines, was in danger, the Transport Ministry said, with automatic switching of navigation to alternative systems.

"As it happened at the time of (military) drills in 2010 and 2011, we suspect North Korea was engaged in jamming signals," a government official said.

Officials at the Korea Communications Commission declined to comment whether North Korea was the source of the signal jamming but said it had been identified as the culprit in at least one similar incident.

A Defence Ministry official declined to comment on the source of the jamming but said the military's equipment had not been affected.

North Korea has stepped up its rhetoric against the South in recent weeks, hurling personal insults at South Korean President Lee Myung-bak and threatening to reduce the capital Seoul to ashes.

It is expected to conduct a third nuclear test any day, possibly using a uranium device which would infuriate neighbouring countries and the United States which have been involved in talks to try to rein in its nuclear weapons programme.

The threat of cyber war from North Korea is seen in the South, one of the world's most wired countries, as increasing in sophistication.

News reports said North Korea operates vehicle-mounted jamming devices that can disrupt signals up to 100 km (60 miles) away and is developing systems with further reach.

(Reporting by Jack Kim and Ju-min Park; Editing by Nick Macfie)

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