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Electronic jamming signals from North Korea which have affected hundreds of civilian flights in South Korea were continuing unabated on Thursday, officials said, amid simmering cross-border tensions.

"GPS (global positioning system) jamming signals are continuing today," Yang Chang-Seang, a transport ministry assistant director, told AFP.

As of 2 pm (0500 GMT), a total of 337 aircraft since Saturday had reported the failure of GPS signals, he said. "But it poses no threat to navigational safety."

The state Korea Communications Commission said the interference was coming from a city just north of the border.

"We've traced the jamming signals to the direction of Kaesong," said Lee Kyung-Woo, a commission deputy director.

The transport ministry said aircraft had GPS signals jammed while flying over the central area of the Korean peninsula, or while taking off from or landing at Incheon or Gimpo international airports near Seoul.

Officials say planes can use other navigation devices like the very-high-frequency omni-directional range (VOR) and inertial navigation systems.

The reason for the jamming, which the North has not admitted, was unclear. But cross-border tensions are high and Seoul is on alert for border incidents after repeated threats by the North against the South's conservative rulers.

It accuses them of defaming Pyongyang's ruling dynasty during mass celebrations last month marking the centenary of the birth of founding leader Kim Il-Sung.

Seoul officials said the money spent on a failed rocket launch and other anniversary events could have been used to alleviate acute food shortages.

The North is widely thought to be preparing for another nuclear test following international criticism of the abortive launch on April 13.

A South Korean nuclear expert has said it has apparently finished preparations and is only awaiting a political decision to go ahead.

Pyongyang said its rocket launch was intended merely to launch a peaceful scientific satellite, while Washington and its allies saw a disguised ballistic missile test.

The UN Security Council strongly condemned the launch and tightened sanctions. On Wednesday it ordered a freeze on the assets of three North Korean state firms said to have financed and organised missile and nuclear programmes.

It was unclear how long the jamming would continue. In March last year GPS jamming signals from the North lasted for 10 days during an annual US-South Korea joint military drill, Lee said.

Seoul's Yonhap news agency at that time said the jamming caused minor disruptions to military phones and navigational devices near Seoul.

In October 2010 the South's then-defence minister Kim Tae-Young said a Russia-made North Korean jamming device capable of disrupting guided weapons posed a fresh threat to security.

He blamed the North for intermittent GPS failures on naval and civilian craft along the west coast from August 23 to 25 that year.