The North's officials at border areas refused to accept a letter of protest sent from Seoul's telecommunication agency, the South's unification ministry handling cross-border relations said.

"The officials said they couldn't accept the letter, without explaining why," the ministry spokeswoman told AFP.

Seoul's defence ministry this month accused the North of sending signals across the tense border to trouble military communications during an annual US-South Korea joint military exercise.

"The North's jamming of GPS signals causes inconveniences among our people, poses threat to their safety... and is an unacceptable act in breach of international customs," the Korea Communications Commission said in a statement.

The agency said the letter urged Pyongyang to "stop the attempts immediately and prevent recurrence".

South Korean mobile users complained of bad connections and the military reported GPS (Global Positioning System) navigational devices malfunctioning on March 4.

The North calls the military exercise a rehearsal for invasion.

South Korea's then Defence Minister Kim Tae-Young said last year the North's jamming devices, modified from Russian technology, were capable of disrupting guided weapons and they posed "a fresh security threat" to the South.

Cross-border tension has been acute since the North's alleged sinking of a Seoul warship that killed 46 sailors in March 2010 and the shelling of a border island that left four South Koreans dead in November.

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