SEOUL (Reuters) - In air conditioned bunkers and at military bases across South Korea, it is with keyboards - not tanks - that South Korean and the U.S. forces will launch military exercises on Monday, denounced by North Korea as a rehearsal for war.

The Aug. 21 to Aug. 31 exercises involve computer simulations designed to prepare for the unthinkable: war with nuclear-capable North Korea.

The wargames, details of which are a closely guarded secret, simulate military conflict with the isolated country. The U.S. describes them as “defensive in nature,” a term North Korean state media has dismissed as a “deceptive mask”.

“The drills deal with all the steps involved in a war, of course, towards victory,” said Moon Seong-mook, a retired South Korean brigadier who regularly participated in the drills until the mid-2000s.

Far from the dusty firing ranges just south of the heavily fortified border with North Korea, U.S. and South Korean troops hunch over laptops and screens wearing earphones and camouflaged combat uniforms, according to photos of past UFG drills on the United States Forces Korea website.

The U.S. military describes the software behind the drills as “state-of-the-art wargaming computer simulations”. There will be no field training during the exercise, according to U.S. Forces Korea.

As part of the exercises, imagery from military satellites orbiting above the Korean peninsula, is at times used to peer deep into North Korea, said a former South Korean government official who declined to be identified.

Banks of monitors and computer graphics create simulated battlefields, complete with troop movements, according to Park Yong-han, a military expert formerly with the state-run Korea Institute for Defence Analysis.

“You can expand a certain area to see what troops are in what sort of status and where they will move,” said Park.

“In the case of North Korea, we can’t see everything in real time but the military deduces the locations of North Korean troops, including the leadership during the exercise”.

That focus on the North Korean leadership is what particularly infuriates Pyongyang, experts say.

“We cannot stand the fact the enemy tries to form schemes to assassinate our leadership,” North Korea’s state news agency, KCNA, said in July.

“We will follow to the ends of the earth those who dare try to harm our core.”

COMMANDO RAID

North Korea’s rapid progress in developing nuclear weapons and missiles capable of reaching the U.S. mainland has fuelled a surge in tension.

U.S. President Donald Trump warned that North Korea would face “fire and fury” if it threatened the United States. The North responded by threatening to fire missiles towards the U.S. Pacific island territory of Guam.

The North later said it was holding off firing towards Guam, while it waited to see what the United States would do next.

Called Ulchi Freedom Guardian (UFG), the joint drills have their roots in a 1968 raid on South Korea’s Blue House presidential complex, when Unit 124 of the North Korean army secretly entered South Korea and unsuccessfully attempted to assassinate the then president, Park Chung-hee.

The United States had been conducting regular “command and control” drills in the years following the 1950-53 Korean War, but combined exercises with the South Korean military following the failed raid, in which all but two of the North Korean commandos were killed.

The United States has about 28,000 troops in South Korea. Many of them will be joining thousands of South Korean forces in the exercise.

Other South Korean allies are also joining this year with troops from Australia, Britain, Canada, Colombia, Denmark, the Netherlands, New Zealand taking part.

“It’s to prepare if something big were to occur and we needed to protect ROK,” a U.S. military spokeswoman, Michelle Thomas, said, referring to South Korea by the initials of its official name, the Republic of Korea.

North and South Korea are still technically at war with the North after the Korean War ended with a truce, not a peace treaty.

China, North Korea’s main ally and trading partner, has urged the United States and South Korea to scrap the drills and so has Russia.

The United States has not backed down.

“My advice to our leadership is that we not dial back our exercises,” said Joseph Dunford, chairman of the U.S. Joint Chiefs of Staff on Thursday in Beijing.

“The exercises are very important to maintaining the ability of the alliance to defend itself”.