SINGAPORE — Will the world's leading powers act in a more united way on Iran's nuclear program than they have so far over climate change?

The answer will become clearer in the next few weeks. It is a key test of U.S. policy to engage China as a long-term partner on major international issues, including many that shape stability in Asia. From the early days of his administration, U.S. President Barack Obama has lobbied Chinese leaders over Iran. He has urged them to join the United States and its allies in increasing pressure on Tehran if it continues to defy resolutions of the U.N. Security Council by refusing to suspend its uranium enrichment program.

Israel says Iran is dangerously close to the point where it may be able to start making nuclear weapons and eventually fit them to the nose cones of missiles that can strike Israel and parts of Europe.