Seoul confirmed Monday that North Korea has been trying since Friday to jam communications signals across the border, where the US and South Korea are holding a major joint military exercise.

Signals are being emitted from near the North's border city of Kaesong to disrupt navigational devices using GPS (the Global Positioning System) northwest of Seoul, the Korea Communications Commission said.

They caused minor inconvenience on Friday and Saturday, it said, while weaker signals are ongoing.

"Intermittent (GPS) disruptions are still continuing, although signals are weak," the commission said in a statement, adding that it was working with government agencies and security authorities to counter the jamming.

The South's defence ministry confirmed the intermittent failure of GPS receivers last week, but refused to give details for security reasons.

It was not clear whether the disruption caused problems to the war games.

The North's military operates dozens of bases equipped for an electronic war to disrupt South Korean military communications, the South's Yonhap news agency said.

The communist country has imported GPS jamming devices from Russia, while South Korea uses French equipment to disrupt or monitor the North's military communications systems, it said.

The South's then-defence chief said last October that the North was capable of interfering with GPS reception over a distance of up to 100 kilometres (60 miles).

He said this was seen as a fresh security threat, since it could potentially disrupt guided weapons such as missiles.

The jamming on Friday and Saturday coincided with cyber-attacks on the websites of about 30 key government agencies and financial institutions in South Korea. The origin of those "distributed denial-of-service" (DDoS) attacks is not known.

A DDoS attack often uses viruses planted in "zombie" computers. These seek simultaneous access to selected sites and swamp them with traffic.

The Korea Communications Commission said more than 77,200 zombie computers were mobilised for the latest attacks, and viruses had destroyed the hard discs of 114 of them.

But the damage was minimised by anti-virus software, it said.

In July 2009 a cyber-attack temporarily shut down 25 sites domestically and in the United States. The South's spy agency has blamed the North for the incident, although US officials reached no conclusion.

Kim Heung-Gwang, a defector who lectured on computer technology at a North Korean college, said Pyongyang might have launched last week's GPS jamming and DDoS attacks to test the South's systems.

"South Korea must make thorough preparations as North Korea may launch far stronger and unexpected attacks next time," he told Munhwa Ilbo newspaper.