The meeting of senior officials from permanent members of the United Nations Security Council, Britain, China, France, Russia and the United States, plus Germany was due to take place on Monday in Brussels, the source said.

But now it has been shelved and no new date has been set for another meeting, they added.

"It was due to be Monday in Brussels but unfortunately, the Chinese were unable to attend," the source said, speaking on condition of anonymity.

On Thursday, Washington -- which charges Tehran is trying to build a nuclear bomb and has been pushing hard for tougher action against Iran -- warned China not to block fresh United Nations sanctions against Iran.

"I don't think China would want to be in a position to cause a failure of democracy to deal with this issue," US envoy to the United Nations Zalmay Khalizad told reporters.

State Department spokesman Sean McCormack also said that no date had been set for the meeting due to "Chinese reticence".

"I can't tell you the reasons behind the reluctance now. We certainly hope that that is temporary and we look forward to working with them to move forward the diplomatic track," he added.

China, which has a Security Council veto and strong trade ties with Iran, is wary of imposing more sanctions, as is Russia.

And relations with the United States were strained last month when President George W. Bush held talks with the Dalai Lama, Tibet's exiled leader, and then attended a Congressional ceremony to award him a medal.

On Thursday, the UN's nuclear watchdog the International Atomic Energy Agency (IAEA) published a mixed report into Iran's nuclear programme, which the officials had been expected to discuss at the meeting.

It said Iran had taken important steps in revealing the extent of its nuclear programme, which it says is for civilian purposes, but was still defying UN demands that it suspend enrichment.

IAEA officials also confirmed for the first time that Iran had installed 3,000 uranium enriching centrifuges, reaching a key milestone in the capacity to produce enough highly-enriched uranium for weapons.

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