South Korea suspects its rival northern neighbour may disrupt the GPS signals of the military when the joint exercises between Seoul and Washington are underway. The warning, amid escalating tensions on the Korean peninsula, comes when the South and the US prepare for its largest-ever joint military drill.

With an existing warning about possible terror attacks emerging from Pyongyang in place, the latest threat is only bound to worsen the situation. "We are preparing for the possibility that around the time [South Korea and the US stage] the Key Resolve drills, the North will discharge electric waves to disrupt GPS signals [in the South]," a Seoul official, who does not wish to be identified, told Yonhap news agency.

The North had earlier beamed powerful broadcast signals disrupting GPS signals in South Korea on at least three occasions in recent years. Mobile phones and other electronic devices experienced a brief outage during those attacks.

"Military equipment containing commercial GPS systems could be affected by the electric waves," the source added. North Korea is known to have strong cyber-warfare capabilities, with thousands of military personnel in the isolated country.

The threat has come when Washington said the upcoming annual exercises with South Korea will be the largest ever staged. The annual military drills, codenamed Key Resolve and Foal Eagle, will begin in March. The US will send 15,000 troops to take part in the manoeuvres, against the 3,700 soldiers which were sent last year.

Key Resolve, a computer-simulated exercise, will last for about 10 days while the Foal Eagle will go on for 40 days. Both the drills usually kick off simultaneously.

In the aftermath of the North's nuclear test and long-range rocket launch, the US has been engaged in a strong and unusual show of force against the Kim Jong-un regime. Besides deploying a nuclear-powered submarine, the US forces have also flown four F-22 stealth Raptors above the Korean peninsula. It is a rare act for the US to send four of those sophisticated fighters at the same time.