An upcoming meeting by five world powers on trying to curb Iran''s nuclear program has been canceled at China''s request, senior officials from three of the countries involved said Monday.

One of the officials said China cited scheduling problems in asking for the cancellation, and the five now plan to talk by conference call. That call was tentatively set for Dec. 22.

The official said China seemed to have genuine problems in attending the meeting in Brussels or outside the Copenhagen climate summit and did not appear to be seeking to delay it. Still, the development was a setback in efforts to present a unified front on Iran in the face of continued Iranian defiance on its nuclear program.

Because it relies on Iran for gas and oil, China is the weakest link in international attempts to punish the country for defying a U.N. Security Council demand that it stop enriching uranium, a process that can make both nuclear fuel and the fissile core of warheads.

The world powers also have to worry about an increasingly edgy Israel. The Jewish state sees an Islamic Republic with such weapons as an existential threat — and has repeatedly indicated it is ready to hit Iran militarily.