S. Koreans warned about military buildup in North. Questions raised on whether buildup constitutes threat

In one of its most forceful warnings to date, the South Korean government has told the nation not to let down its guard against a North Korean military threat. Defense Minister Lee Ki Baek detailed a massive buildup of North Korean military forces and called for national unity to ward off an attack that could come by the end of the decade.

The call comes at a time when internal political tensions are running high. The opposition has launched a controversial campaign to revise the Constitution in the face of stiff government resistance. Mr. Lee seemed to anticipate criticism that his message would be interpreted as a cynical attempt of the government to justify political oppression by invoking a national threat.

``Due to the fact that, in the past, attempts were made to take advantage of our security situation and the people's security consciousness to bolster a prolonged hold on power,'' he said, ``there seem to have developed among some citizens a tendency to be skeptical about our security needs and thus to entertain a false sense of security.''

Lee's plea to South Koreans to refrain from acts that might cause ``internal division or social confusion'' is a clear message that the military firmly backs President Chun Doo Hwan's call for a moratorium on political debate until 1989 -- after the 1988 Seoul Olympics and after Mr. Chun's term of office expires.

The defense minister's description of the buildup of North Korean forces is backed by the reports of United States military analysts. The US maintains about 40,000 troops in the South to deter an attack by the North and is committed by treaty to South Korea's defense.

Since the summer of 1985, North Korea has acquired new offensive weapons from the Soviet Union, including MIG-23 aircraft and ground-to-ground missiles. Over the past year, North Korea has gradually raised the level of troop concentration in front-line areas from 45 percent to 65 percent of its total military strength. The effect is to reduce reliable early-warning time for an attack from several days to several hours.

US analysts have repeatedly said that a North Korean attack could be countered effectively with sufficient warning. Now there is the worry that, in a sudden attack, North Korean forces could punch through South Korean and US defense lines and gain a decisive advantage in five to seven days of intense warfare before US reinforcements could arrive.

``North Korea could march down to the Han River [which runs through metropolitan Seoul] and then call for a truce,'' says a government official. ``Under the current international climate, there would be great international pressure to negotiate, and North Korea would have already won the war.''

The North has developed extensive capabilities to infiltrate South Korean lines with the use of low-flying planes, helicopters, and small submarines that are difficult to detect.

According to analysts here, North Korean forces (more than 800,000 strong) enjoy a 20-percent manpower advantage, a 2.7-fold advantage in tanks, a two-fold advantage in field artillery, and 40 percent superiority in aircraft numbers.

US analysts take a somewhat less excited view of the military balance. Although they agree that the North Korean buildup is alarming because of its offensive nature, they say that the South would enjoy a key advantage in air superiority, with its sophisticated F-16 aircraft. Air superiority proved decisive in the Korean war in the early 1950s because of the rough terrain that made armored warfare difficult.

North Korea would also have to consider the possibility that the US would use nuclear weapons in any conflict. ``The North should never miscalculate our ability to respond,'' says a senior US military official.

South Korean leaders have long warned that the South faces a dangerous period in the coming years. The country's rapid economic development will allow it easily to surpass the North in military strength in the early 1990s -- which, by most calculations, leaves the North only a few more years of military advantage.