"The Ulchi Focus Lens exercises began today as scheduled to run until September 1," a South Korean defense ministry spokesman said.

The drills, which involve a massive computer-simulated war game, have been denounced by North Korea as a prelude to war against the communist country.

But Seoul and Washington deny the North Korean claims, saying the exercises are "defense-oriented" and have been held annually since 1975.

"They are purely defensive, defense-oriented drills," said Kim Yong-Kyu, a spokesman of the US-South Korea Combined Forces Command in Seoul.

The exercises largely feature computer-simulated war games conducted at command posts and not field maneuvers, he said.

Some 4,000 US troops -- more than the 3,000 initially announced -- were brought in from the Pacific and elsewhere to join another 5,000 US troops, part of the American contingent permanently stationed here, he said.

South Korea, which is seeking rapprochement with North Korea, refuses to disclose the number of its troops taking part in the military maneuvers.

Pyongyang's official Korean Central News Agency (KCNA) last week again condemned the joint exercises as "grave military provocation".

"They are nothing but a very dangerous military adventure driving the situation of the Korean peninsula to the brink of war," KCNA said Friday.

North Korea, in defiance of international warnings, test-launched seven ballistic missiles into the Sea of Japan (East Sea) on July 5. The UN Security Council unanimously condemned the move and imposed sanctions on Pyongyang.

In protest at the UN measures, North Korea warned it could take "stronger physical actions" in response.

US television network ABC said Thursday Pyongyang may be preparing an underground nuclear test. The governments in both Seoul and Washington remain cautious about the authenticity of the report.

ABC said a US intelligence agency had recently observed "suspicious" activities, including the unloading of large reels of cable outside a suspected underground test site called Pungyee-yok in northeast North Korea.

North Korea announced in February 2005 that it had manufactured nuclear weapons but nuclear weapons tests have never been reported.

North Korea has boycotted six-way nuclear disarmament talks -- also including China, the United States, South Korea, Japan and Russia -- since November, citing US economic sanctions.

Seoul said Sunday it was stepping up its monitoring of North Korean nuclear activities, admitting it had stationed military personnel at a state seismology institute capable of detecting underground nuclear tests.

Some 20 anti-war activists staged a protest rally on Monday in front of the main US military base in downtown Seoul, demanding the military exercises stop.

"Stop the military training for invading North Korea! Pull the US military troops out of South Korea!" the protesters chanted.

The US military presence in South Korea dates back to the bloody 1950-1953 Korean War and was cemented by a mutual defense treaty signed afterwards.

In a major change to the military alliance, the United States is reducing its forces in South Korea from 37,000 two years ago to 25,000 by 2008.

South Korea is pushing to secure wartime control over its troops which are currently under a US-led combined command.

US troops are stationed in the South to help its 650,000-strong army face up to North Korea's 1.2 million-strong army.

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