North Korea used jamming equipment to block South Korean military communication devices last week, a report said today, amid high tension over the joint drills between Seoul and Washington.
Yonhap news agency said strong jamming signals sent across the border on Friday had caused minor disruptions to phones and navigational devices using GPS (Global Positioning System) at military units near the capital Seoul.
The signals are believed to have been sent from the North's military facilities in Haeju and Kaesong close to the heavily-fortified border, it said, citing Seoul intelligence and military officials.
"The signals were sent intermittently every five to 10 minutes... we suspect the North was testing new GPS jamming devices imported from overseas," said an intelligence official quoted by Yonhap.
"We are preparing systems to control and overcome such jamming signals," it quoted another official as saying.
A defence ministry spokesman declined to comment.
South Korea's former defence chief Kim Tae-Young said last year a device the North had that was capable of disrupting guided weapons posed "a fresh security threat" to the South.
Kim said Pyongyang was thought to have been behind the intermittent failure of GPS receivers on naval and civilian craft along the west coast during the joint military exercise between the South and the US last August.
The North had modified Russian equipment to make its own jamming devices, he said, warning the communist country was capable of interfering with GPS reception over a distance of up to 100 kilometres (60 miles).
The North's GPS interrupter is believed to be effective in preventing US and South Korean guided bombs and missiles from hitting their target accurately.
Cross-border tension has escalated since last week when Seoul began the regular Key Resolve/Foal Eagle military drills with the US, which Pyongyang labelled as a rehearsal for invasion.
The communist country, which habitually criticises joint military manoeuvres between the two allies, threatened last week an "all-out war" and "physical counter-action" on the drills staged south of the border.
Inter-Korea ties have been icy 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.