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.

A senior intelligence official here said that "the international community has not so much leverage" over North Korea now, in part because it cannot further isolate one of the world's most isolated nations.

The South Korean Government, which has been eager not to press the North too hard by moving to economic sanctions that could prompt North Korea to lash out, said it viewed the developments "in a positive light." But to resolve the problem, its Foreign Ministry said in a statement, "the North has to realize its safeguard agreement with the International Atomic Energy Agency" and move ahead with the mutual inspections that North and South Korea agreed on in 1991. In Japan, news that the North had suspended its withdrawal from the treaty was overshadowed by detailed, if still somewhat murky, accounts of the missile test.

The Japanese official who described the test insisted that he not be named, but he often acts as a spokesman for Japanese prime ministers. The official said that Japan's Defense Ministry "was very much shocked" by the test, in which a missile was launched from a base near Wonsan, on the southeastern coast of North Korea. Test Target a Buoy

According to some accounts, it was aimed at a target buoy floating in the Sea of Japan, and flew in the direction of the Noto Peninsula of Japan. Newspapers published a photograph today of a North Korean frigate in the region on the Sea of Japan, apparently taken on May 29.