While North Korea's negotiators were dragging out the standoff over a nuclear non-proliferation treaty that ended on Friday at the United Nations, the Communist Government's engineers conducted what appears to be the first successful test of the country's homegrown mid-range missile, one that Japanese officials fear could travel as far as some of Japan's most populous cities.

The tests, conducted on the Sea of Japan on May 29 and 30, are believed to have involved the Rodong 1, a missile North Korea has been developing for several years and is preparing to export to Iran in return for oil. American intelligence officials have said that the missile is believed to be capable of carrying a payload of chemical weapons, or perhaps a small nuclear device.

Although the test was apparently monitored by both American and Japanese officials, it was not revealed until Friday. The test was described by a senior official in the office of Prime Minister Kiichi Miyazawa who briefed Japanese reporters just hours before North Korea averted a major confrontation with the United States and its allies by announcing that, at least for now, it would not withdraw from the Nuclear Non-Proliferation Treaty.

North Korea had initially scheduled its withdrawal from the treaty for today. It would have been the first time any nation pulled out of the pact.

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While President Clinton called North Korea's decision to defer withdrawal from the treaty a "first but vital step," officials in Japan and South Korea said privately that it had solved little. After three months of talks centering on getting North Korea to recommit itself to the treaty, one senior Japanese official said today, "things have only returned to the point where they were in March." The key nuclear sites that may reveal how much plutonium the Pyongyang Government has produced remain closed to international inspectors, he said, and "Most importantly, we still think they are moving forward with their weapons project," he added.