Iran sent mixed signals Thursday on how it will respond to the world's incentives to give up uranium enrichment, with its supreme leader vowing his country would never back down, while the president and chief envoy to the U.N. nuclear watchdog said Tehran was prepared to negotiate.

The Iranians spoke as American and European officials in Vienna urged Tehran to freeze enrichment and stop withholding information about its nuclear program. The chief U.S. delegate to the International Atomic Energy Agency, Gregory L. Schulte, warned that if Iran rejected the incentives, it could face "the weight of the Security Council."

But supreme leader Ayatollah Ali Khamenei dismissed such talk.

"The Islamic Republic of Iran will not succumb to these pressures," state television quoted Khamenei as saying.

Speaking to Iranian nuclear experts, Khamenei said the development of nuclear technology was more important than oil extraction — the source of about 80 percent of Iran's foreign exchange.

"Let me tell you, the importance of achieving and using nuclear energy is higher than oil exploration for our country," Khamenei said.

The five permanent members of the U.N. Security Council, plus Germany, have offered Iran a package of incentives in return for a long-term moratorium on enrichment — a process that can produce fuel for nuclear power plants or material for atomic weapons. The package calls on Iran to suspend enrichment for the duration of any negotiations.

Iran has not responded formally so far, but its officials have insisted that enrichment is an inalienable right and that talks must be unconditional.

The country denies accusations by the U.S. and others that it is seeking to develop nuclear weapons, saying its program would only generate energy.

White House spokesman Tony Snow said he fully expected Iran to try any number of ways to shape the debate over its nuclear program.

"I think it is safe to say they will try to test the unity" of the United States, Britain, France, Russia, China and Germany, Snow said.

Russian President Vladimir Putin said Iran was prepared to negotiate on the basis of the incentives.

Speaking after talks with Iranian President Mahmoud Ahmadinejad in Shanghai for a regional summit, Putin said: "The Iranian side responded positively to the six-nation proposal for a way out of the crisis."

He added he hoped Iran would soon set a date for the start of talks.

Ehsan Jahandideh, a member of Ahmadinejad's delegation in Shanghai, confirmed the president had offered to negotiate "to ease tensions."

In Vienna, the chief Iranian delegate to the IAEA, Ali Ashgar Soltanieh, warned the West against threatening Tehran, saying "the carrot and stick has always been counterproductive."

But Soltanieh told reporters Iran was "determined to ... find a negotiated solution."

He spoke after the Europeans and the U.S. had criticized Iran before the 35-nation IAEA board.

"Cooperation with the agency has been reduced to almost nothing these last few months and ... numerous important questions remain to be resolved," said chief French delegate Francois-Xavier Deniau, who was speaking on behalf of France, Britain and Germany.

Schulte agreed, saying, "Iran continues to withhold cooperation with the IAEA on almost every outstanding issue."

When Iran received the incentives last week, it said they contained "positive steps" but also ambiguities, which had to be clarified in further talks.

The package included some significant concessions by the U.S., aimed at enticing Tehran to freeze enrichment. The U.S. would provide Iran with peaceful nuclear technology, lift some sanctions and join direct negotiations with Tehran.

The package also pulls back from demands that Iran outright scrap its enrichment program as an initial condition for negotiations, seeking instead a suspension. However, it also contains the implicit threat of U.N. sanctions if Iran remains defiant.