Secretary of State Condoleezza Rice (search) is signaling that Iran must move quickly to accept economic incentives to abandon its suspected nuclear arms development program or risk harsh U.N. sanctions.

Rice commented Friday as the United States softened its public posture and agreed to support an incentive program. Bush administration officials privately expressed skepticism that Iran would live up to the bargain.

Until now, the administration has insisted that Iran deserves no reward for simply abiding by an international arms compact that forbids nuclear weapons development. The United States suspects Iran is using a legitimate program to develop nuclear power plants as cover for illegal weapons development.

"I'm pleased that we are speaking with one voice with our European friends," President Bush said during a trip to Shreveport, La. "I look forward to working with our European friends to make it abundantly clear to the Iranian regime that the free world will not tolerate them having a nuclear weapon."

The United States agreed to drop opposition to Iranian membership in the World Trade Organization and to allow some sales of spare parts for civilian aircraft. If that carrot does not work, the Europeans agreed to support use of the stick the United States has unsuccessfully sought before: U.N. sanctions.

Rice said there is no timetable for negotiations, but added, "This has been going on for some time."

"I would think that if the Iranians are going to demonstrate that they are prepared to live up to their obligations, that they would want to do that sooner rather than later," she told reporters after meeting with Ukrainian Foreign Minister Borys Tarasyuk (search).

"The Iranians need to take the opportunity that the Europeans are presenting them," Rice said.

The European Union warned Tehran explicitly about possible U.N. action Friday.

"We shall have no choice but to support referring Iran's nuclear program to the U.N. Security Council," a confidential EU document obtained by The Associated Press said.

There was no immediate response from Tehran.

Vice President Dick Cheney (search) said concern remains that Iran wants the capability to "enrich fuel far beyond what's required for a civilian reactor to levels that would give them the capability to build a weapon."

"If the Iranians don't live up to their obligations and their international commitments to forgo a nuclear program, then obviously we'll have to take stronger action," Cheney said in an interview on Fox News Channel.

It remains an open question whether Iran will surrender its right to both enrich uranium and reprocess it, said a senior administration official who briefed reporters on the condition of anonymity.

Still, the official said, the United States has made a move that could help the Europeans in their negotiations with the Iranians.

The shift places the United States side by side with British, French and German diplomats. Just weeks ago the administration had seemed to write off their talks with Tehran as fruitless.