US intelligence analysts have discovered evidence that South Korea is trying to develop longer-range ballistic missiles while keeping some of the program's key aims secret from Washington, US officials say.
US spy satellites detected fresh evidence of the program's extent last year, and US concerns intensified further after a missile test this year, the officials said.
The US, South Korea's closest ally, has been tracking its missile research carefully for years.
US President Bill Clinton and his top aides discussed their latest concerns with top South Korean officials last summer.
In addition, the Clinton administration has been pressing North Korea to restrain its missile programs.
The spy satellite photos revealed last year indicate that South Korea had built a rocket motor test station without notifying the US, according to Pentagon analysts who reviewed the intelligence.
The station, which includes a large concrete or tempered steel cradle, in which rocket motors are locked for firing tests, appeared to have been built secretly as part of a larger South Korean ballistic missile program, the officials said.
In April, South Korea conducted a short flight test of a new missile that appeared to violate its agreements with the US, American officials said.
For Clinton administration officials already deeply worried about North Korea's missile and nuclear programs, South Korea's apparent efforts to develop a strategic capability of its own have raised the prospect of a regional arms race at a time when North Korea's stability is increasingly in doubt.
South Korea's missile ambitions prompted Clinton to discuss the issue personally with South Korean President Kim Dae-Jung in July, US officials said, and US Defense Secretary William Cohen also raised the issue during a trip to Seoul later that month.
Clinton administration officials refused to comment directly on the evidence uncovered by the US spy satellites last year, but one acknowledged that parts of the South Korean ballistic missile program have been hidden from the US.
``We are interested in greater transparency in their missile development,'' the official said. ``We don't want this issue to become a point of friction in our bilateral relationship.''
The officials stressed, however, that no single piece of intelligence had suddenly prompted the administration's efforts to limit South Korea's missile program. Washington has known about South Korea's efforts to develop ballistic missiles for years, they said, and the Clinton administration has been working quietly but consistently to contain the program.
``This is an issue of longstanding concern between us,'' said one US official. ``They have been working on a ballistic missile program for a long time, and this is an issue that is raised with them frequently.''
A South Korean government official, however, denied that Seoul has sought to shield parts of its ballistic missile program from the US.
``I believe we have kept transparency in the missile field with the United States,'' said Yoon Joe-shim, an official at the South Korean Embassy in Washington.
While no final agreement between the US and South Korea has been reached, the Clinton administration has signaled to South Korea that it is willing to accept limited improvements on the range of South Korea's ballistic missiles, US officials said.
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