Despite international pressure to curtail its missile program, North Korea is building at least two new launch facilities for the medium-range Taepo Dong 1 and has stepped up production of short-range missiles, according to U.S. intelligence and diplomatic sources.

The projects, and a conclusion by U.S. intelligence agencies that North Korea intends to test-fire a second missile capable of striking Japan, are inflaming regional tensions, U.S. officials and Korea experts said.

Japan is responding to the North Koreans by seeking to boost its defenses with an antimissile system, a move seen by China as upsetting the military balance in the region.

President Clinton, who arrives in South Korea today for a state visit, is scheduled to address the missile issue with his hosts as part of discussions on security matters. The administration also is trying to keep alive a nuclear agreement with Pyongyang at a time when U.S. policy toward North Korea is facing criticism as ineffectual.

A high-level State Department delegation completed a visit to North Korea Wednesday after being denied access to a huge underground construction complex that U.S. intelligence officials believe is a nuclear facility. U.S. officials rebuffed a request by Pyongyang for a $300 million payment in exchange for permission to enter the complex.

The public focus during Clinton's trip and that of the State Department group, led by special envoy Charles Kartman, has been on Pyongyang's nuclear program, which North Korea promised to freeze under the 1994 Framework Agreement. But U.S. officials and other experts say the missile program is on the agenda because it is a more imminent threat.

"The administration certainly has serious concerns about the missile program," said White House foreign policy spokesman David Leavy. "This issue {is} an important element of the president's agenda in Japan and South Korea. We are going to raise this and make the security issues an important part of this trip."

"The missile threat is much more immediate," said David Kay, chief U.N. nuclear weapons inspector from 1991 to 1992. "These are weapons of terror and intimidation."

Like the nuclear issue, there appears to be no resolution of the missile problem in sight. The last bilateral missile talks between the United States and North Korea took place more than a month ago, and no future talks are set. At the time, Pyongyang reportedly asked Washington for up to $1 billion a year for three years to convert its missile production plants. Last year, the United States offered to ease some long-standing economic sanctions on North Korea in exchange for restraint on missile exports, U.S. officials said, but no agreement has been reached.

On Aug. 31, North Korea launched a test of the three-stage Taepo Dong 1, with a range of approximately 1,250 miles, over Japan. Its first stage splashed down in the Sea of Japan and a second stage crossed over Japan's main island of Honshu and landed in the Pacific Ocean. A third stage, which U.S. intelligence sources had failed to detect until reviewing data several weeks later, broke into pieces and traveled 3,450 miles downrange.

Intelligences sources said North Korea is building a launch facility for the Taepo Dong 1 at Yongo dong that could be completed in 1999. Workers have completed bunkers for propellant fuel and are constructing a launch platform. A similar site is under construction at Chiha-ri, the technical support base for North Korea's Scud missile brigade, sources said.

"We have identified some construction that we think might be bunkers to store Taepo Dongs in," said a senior U.S. official. "You could roll them out and elevate them into a position to launch."

North Korea also has recently stepped up production of its No Dong short-range missile, which U.S. officials believe are destined for export. U.S. officials said the missiles are likely to be sold to Syria, Pakistan or Libya, each a prior customer.

On paper, the ballistic missile and nuclear issues are separate. North Korea has publicly proclaimed its intention to build, launch and market intermediate and long-range missiles to raise revenue for its shattered economy and to launch satellites.

"They are proliferator number one, selling not only missiles but production capability" to countries such as Iran, according to an administration official who monitors the North Korean program. Pyongyang is under no legal or treaty obligation to restrain missile development or sales.

On the nuclear side, however, North Korea is bound by the 1994 deal with the United States known as the Framework Agreement. In that pact, North Korea agreed to suspend operations at its known nuclear weapons site at Yongbyon -- halting the separation of plutonium and suspending operations of a plutonium-producing nuclear reactor -- in exchange for the donation of two light-water nuclear power plants. The United States also pledged to supply 500,000 metric tons of heavy fuel oil annually until the power plants are on-line.

U.S. officials said the nuclear and missile issues are linked because the financing package for the nuclear power plants includes $1 billion from Japan. Washington assumes the Japanese parliament would refuse the appropriation if North Korea fired another missile across Japanese territory. Congress also would be expected to balk at putting up further funds for a deal to which many members are already hostile. The U.S. commitment is $35 million a year, used for fuel oil purchase and transportation.

U.S. officials have said they are convinced that North Korea wants implementation of the agreed framework but is suspicious of the U.S. commitment to it because of delays in shipments of the fuel oil. Washington has urged China to deliver the message to North Korea that further missile launches over Japan would jeopardize the agreement because the Japanese parliament and the U.S. Congress would cut off funding, officials said.

A more immediate threat to the agreed framework, however, is the U.S. belief that North Korea is pursuing nuclear weapons activities in massive underground caverns at Qum Chang. The published text of the agreed framework restricts nuclear activities only at Yongbyon, the known site, but U.S. officials say an unpublished "confidential minute" banned construction of Yongbyon-type facilities anywhere else in North Korea.

"We are going to need inspections of the site or sites that might be involved," Defense Secretary William S. Cohen said yesterday. "Absent . . . agreement to do that, it will call very much into question the agreed {nuclear} framework itself."

U.S. officials acknowledge that they have little leverage over North Korea on this issue other than North Korea's overall desire to improve relations with Washington. Absent of an agreement to curtail missile exports, the United States has set up a system of interdiction to block North Korean missile-parts cargoes from reaching their destinations.

North Korea routinely falsifies cargo manifests for missile exports, U.S. officials said, making it possible for Washington to tip off customs and other law enforcement officials at intermediate ports of call and have the shipments intercepted.