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.

Israeli Defense Minister Ehud Barak urged the world to agree to tough new penalties while again suggesting that military strikes remained an option.

"There is a need for tough sanctions," Barak told reporters in Vienna during an official visit. "Something that is well and coherently coordinated to include the Americans, the EU, the Chinese, the Russians, the Indians."

At the same time, he said, "we recommend to all players not to remove any options from the table," just as "we do not remove it."

In Washington, Secretary of State Hillary Rodham Clinton acknowledged that U.S. administration's nearly yearlong effort to engage Iran has fallen short and new sanctions are needed to press Tehran to provide more information about its suspect nuclear program.

Clinton's pessimistic remarks come as an end-of-year deadline, set by President Barack Obama, looms for the Iranians to prove that their nuclear intentions are peaceful.

She said the administration has offered Iran a chance to participate in meaningful discussions about its nuclear activities and intentions or face fresh penalties for defiance in line with the dual-track, carrot-and-stick approach.

That dual effort, though, has "produced very little," she said, adding that "additional pressure is going to be called for" to get results.

During seven years of failed international diplomacy, Iran has moved closer to being able to make nuclear arms, even while insisting that its atomic program is meant solely to generate energy.