He also denied suggestions that Washington had "caved in" by shelving for two weeks Security Council consideration of a tough draft resolution that would legally require Tehran to halt sensitive nuclear fuel work that could be used for bomb-making.

Washington agreed to allow France, Britain and Germany in the interval to put together a package of "carrots and sticks" to try to lure Tehran away from uranium enrichment.

"The suspension of uranium enrichment and reprocessing remains a red line for the Europeans. It certainly remains a red line for us. We believe it's a red line for Russia and China," the US ambassador said.

"This is a delay but it's intended to show the American willingness to try and exhaust every diplomatic possibility, and it proves again that the key to this still lies in Iran's hands," he added.

Western powers suspect Tehran is using its civilian nuclear program as a cover to seek nuclear weapons, but Iran insists its program is peaceful and has vowed not to suspend its uranium enrichment program.

Bolton said the US stance would be determined by whether the council's veto-wielding members -- Britain, China, France, Russia and the United Nations -- reached consensus on the European package of economic, energy and security incentives for Iran.

Diplomats said negotiators from the Security Council's five permanent members plus Germany planned to meet in London on May 19 to weigh a new package of incentives as well as penalties.

"I am confident that one way or the other there's going to be a resolution along the line we were pushing, because it is important to make mandatory on Iran the requirement of suspending its enrichment-related activities," he noted.

The draft now on hold invokes Chapter Seven of the UN Charter that can authorize sanctions or even military action as a last resort. But its Western sponsors stressed any decision on sanctions at a later stage would require a separate resolution.

Russia and China, which have close trading ties with Tehran, have made it clear they oppose coercive measures to rein in Iran's nuclear activities.

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