NORTH KOREA’S latest nuclear test may mark a new and more risky strategy by a regime headed by a 29-year-old novice. Previous detonations by Pyongyang appeared intended mostly as crude provocations, designed to win credibility at home and concessions from South Korea and the United States. Now North Korea may be aiming to become a full-fledged nuclear power, with warheads and missiles that could threaten its neighbors and eventually the U.S. homeland.

The yield of the blast, as measured by seismographs, suggested it was twice as large as the last North Korean test, in 2009. Potentially even more significant was the regime’s claim that it had built a “smaller and light” bomb with “diverse materials.” In other words, the North may have made progress toward building a miniaturized warhead that could fit atop one of its long-range missiles and may have fabricated a weapon from enriched uranium, rather than the plutonium it has previously used.

Since Pyongyang has a limited supply of plutonium but is operating at least one uranium enrichment plant, development of a uranium bomb would give it the potential to build an arsenal of warheads to go with the long-range missiles it is developing. It will take time for outsiders to learn whether the bomb was actually composed of uranium. But the United States and its allies need to respond to the possibility that the Kim regime is headed down a more dangerous and provocative path.

This should not mean trying once again to engage North Korea in negotiations: More than 15 years of such efforts have demonstrated that the United States lacks the leverage to induce the regime to give up its nukes. If any country has such leverage, it is China, which supplies its neighbor with fuel and food. U.S. diplomacy should be aimed first at pressuring Beijing to take responsibility for the growing menace on its doorstep. New Chinese leader Xi Jinping has the opportunity to change a policy that, in backing the Kim regime in the interest of “stability,” has made the Korean peninsula steadily more dangerous.

Though sanctions on North Korea are already tight, the Obama administration should look for new ways that the U.S. financial system can be used to cut off the regime’s access to international banks. It should work to bring greater attention to the human rights calamity in the North. And it should accelerate work on missile-defense systems. North Korea must get the message that nuclear tests will be answered with sanctions, not concessions. China must understand that a failure to reverse the buildup will lead to a larger arms race in Asia.