SEOUL, South Korea — China, which holds the key to whether tough U.N. sanctions will be imposed, warned North Korea today that its nuclear test would harm relations and called on the United Nations to take "appropriate measures" to get North Korea to relinquish its nuclear weapons.

China's Foreign Ministry vented its anger against its communist ally over the test for a second day, with a spokesman saying that relations had been damaged.

"The nuclear test will undoubtedly exert a negative impact on our relations," the spokesman, Liu Jianchao, said at a media briefing. He said Monday's test was done "flagrantly, and in disregard of the international community's shared opposition."

But Liu urged diplomatic efforts to resolve the crisis and said that the time was not right for punishment, much less military action.

China has been North Korea's major ally and a source of both food and fuel for the desperately poor nation of 23 million. Behind Beijing's largesse is a fear that a collapsing North Korea could bring U.S. troops stationed in South Korea to China's doorstep, or send refugees pouring across the border, destabilizing the Chinese industrial northeast.

"Taking military action against North Korea would be unimaginable," Liu said. "What we should discuss now is not the negative issue of punishment."

"Instead, the international community and the United Nations should take positive and appropriate measures that will help the process of de-nuclearization on the Korean peninsula," he said.

Japan's leader said his nation still had no intention of seeking atomic weapons, easing fears of a new regional nuclear arms race.

U.S. Ambassador to the U.N. John Bolton said ahead of a Security Council meeting today that the standoff was one "between North Korea and the rest of the world," which will result in sanctions or more serious punishment. He refused to rule out military action, including a naval blockade, but emphasized President Bush wants to resolve the matter using peaceful means.

China has long opposed sanctions sought by countries such as the United States, which is pressing for potentially crippling new measures against Pyongyang.

A permanent U.N. Security Council member, China has a decisive say over how stern a punishment the international community can mete out to North Korea. A South Korean envoy, returning to Seoul from Beijing, said today that China appeared to be leaning toward backing strong U.N. measures.