Since March 4, Pyongyang has been trying to disrupt GPS receivers critical to South Korean military communications apparently in protest of the ongoing joint military training exercises between South Korean and U.S. forces. Strong jamming signals were sent intermittently every five to 10 minutes.

The scope of the damage has been minimal, putting some mobile phones and certain military equipment that use GPS signals on the fritz.

Large metropolitan areas including parts of Seoul, Incheon and Paju have been affected by the jamming, but "the situation is getting wrapped up, no severe damage has been reported for the last two days," Kyoungwoo Lee, deputy director of Korea Communications Commission, said.

The jamming, however, has raised questions about whether the Korean peninsula is bracing for new electronic warfare.

The North is believed to be nearing completion of an electromagnetic pulse bomb that, if exploded 25 miles above ground would cause irreversible damage to electrical and electronic devices such as mobile phones, computers, radio and radar, experts say.

"We assume they are at a considerably substantial level of development," Park Chang-kyu of the Agency for Defense Development said at a briefing to the parliament Monday.

Park confirmed that South Korea has also developed an advanced electronic device that can be deployed in times of war.

The current attempts to interfere with GPS transmissions are coming from atop a modified truck-mounted Russian device. Pyongyang reportedly imported the GPS jamming system from Russia in early 2000 and has since developed two kinds of a modified version. It has also in recent years handed out sales catalogs of them to nations in the Middle East, according to South Korea's Chosun Ilbo.

North Korea Jams GPS Signals in Ominous Threat of More to Come

Major Korean newspaper editorials today called the recent jamming a "wake-up call," pointing out that consequences could be severe if North Korea succeeds in discharging full-fledged electromagnetic waves.

On top of disrupting major communication tools used by both civilians and the military, the waves would affect financial transactions and civilian airplanes dependent on radio signals.

"The problem could be further exacerbated by the fact that our military equipment increasingly relies on commercial GPS standards," wrote JoongAng Daily, one of South Korea's largest newspapers.

This is the second time North Korea has sought to interfere with military communications. Pyongyang is thought to have been behind a failure of GPS receivers on some naval and civilian aircraft during another joint military exercise in August.

South Korea's minister of defense at that time had reported to the Congress, warning that the North poses "a fresh security threat" capable of disrupting guided bombs and missiles by sending signals over a distance of up to 60 miles.

Some modern weapons are equipped with an alternative guided system in addition to GPS, which means the bomb would find its way to the target even if it loses contact with the satellite.

But the Korean military weaponry still largely remains vulnerable to GPS jamming signals, said Kwon Oh-Bong of the Defense Acquisition Program Administration, answering questions from concerned politicians at a parliamentary working session Monday.

"Because we have a special code for the military, it is unlikely to be affected by such an attack, but there are some weapons that do not require a special code, so we are researching preventive measures," he said.

U.S. Forces Korea spokesman David Oten declined to assess the effects, saying it is a matter of intelligence but added in an e-mail response that they are conducting extensive analysis of potential threats and ensured that "United States forces operate using multiple, redundant navigational systems and train extensively to operate in a contested electronic environment."

Euri Son and Esther Kim contributed to this article.

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