Seoul has accused North Korea of using sophisticated jamming systems to block South Korean military signals and disrupt its guided missiles.

South Korea's Yonhap news agency said the strong jamming signals have been transmitting from the northern border city of Keasong since last week.

The purported purpose of the jamming was to disrupt navigational devices using Global Positioning Systems as a major joint military exercise is under way northwest of Seoul.

The jamming is believed to have prevented some U.S and South Korean bombs from hitting their targets during a military drill. The Korea Communications Commission said they caused minor inconvenience last week.

"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 shield against the jamming.

The defense ministry also confirmed the jamming but refused to afford details citing security reasons.

It is believed that Pyongyang has modified Russian equipment to make its own jammers, which can interfere with GPS up to 50 miles away.

The U.S.-South Korean annual Key Resolve drill kicked off Feb. 28. More than 12,000 U.S. troops are taking part in the drill, alongside 200,000 South Korean soldiers. The exercise, including live drills and computer simulated war games, is expected to run for 11 days.

South Korea and the United States stage regular exercises with their combined forces. The recent drill, though, comes amid heightened tension with North Korea.

Both Seoul and Washington have invited media to cover several of the drills scheduled for the coming weeks, including when railroads are used to move weapons and the air landing of troop reinforcements from other countries.

Relations between North and South have soured since North Korea's suspected sinking of a South Korean war ship last March and Pyongyang's artillery shelling of a South Korean island in November.

GPS jammers work by sending a signal that interferes with the communication between a satellite and GPS receiver.

South Korea is also purported to use French made equipment to disrupt or monitor the North's military communications systems.

Military officials said the jamming coincided with cyberattacks on the Web sites of about 30 key government agencies and financial institutions in South Korea. The origins of those attacks have yet to be determined.

The Korea Communications Commission said that more than 77,200 "zombie" computers were mobilized for the latest attack. The viruses destroyed the hard disks of 114 of them.