The implied threat was dismissed by U.S. Secretary of State Condoleezza Rice, who said Iran was too dependent on oil revenues to disrupt the flow of crude. She also put Iran on notice that the incentives offered by the West to suspend its nuclear program are not open-ended, although she declined to say Tehran had a firm deadline to respond.

Khamenei, who has the final say on all state matters, made his comments in a speech broadcast live on state radio Sunday.

"If you make any mistake (punish or attack Iran), definitely shipment of energy from this region will be seriously jeopardized," Khamenei said, addressing Western nations.

Khamenei said the United States and its allies would be unable to secure oil shipments passing out of the Gulf through the strategic Strait of Hormuz to the Indian Ocean. At its narrowest point, the strait separating Iran from the Arabian peninsula is 44 miles wide.

"You will never be able to protect the energy supply in this region. You will not be able to do it," he said.

Khamenei, however, did not specify how oil supplies would be disrupted, and he insisted to the assembled throng that Iran would not be "the initiator of war."

In a television interview later Sunday, Rice sought to play down Khamenei's remarks.

"I think that we shouldn't place too much emphasis on a threat of this kind," she said on "Fox News Sunday." "Obviously it would be a very serious problem for Iran if oil were to be disrupted on the market."

Last week, Rice said the United States was prepared to join the European Union and Germany in negotiations with Iran only if Tehran agreed to stop enriching uranium. The Western nations fear Iran is using what it calls a peaceful civilian nuclear program as a cover to build atomic weapons.

Khamenei said Iran was not a threat to any country.

"We have not threatened any neighbor," he said, calling the accusation that Iran is seeking an atom bomb "a sheer lie."

"It's against Islamic teachings," he said.

However, Iranian President Mahmoud Ahmadinejad repeatedly has questioned Israel's right to exist and said in October the Jewish state should be "wiped off the map." Israel is believed to possess the world's sixth-largest nuclear arsenal.

Khamenei's harsh rhetoric came a day after Ahmadinejad said a breakthrough in negotiations was possible. He welcomed the U.S. offer to join talks but rejected preconditions. Ahmadinejad attended Khamenei's speech Sunday.

Contrary to Khamenei's remarks, other Iranian officials have repeatedly ruled out using oil as weapon. Iran is the world's fourth-largest oil exporter and has the second-largest reserves in the Organization of Petroleum Exporting Countries.

Iranian political analyst Saeed Leilaz said Khamenei's remarks were more important for what he did not say.

"He didn't close the door for dialogue or understanding with the U.S.," Leilaz said. "Khamenei even didn't close the door for possibility of Iran suspending uranium enrichment. Iran is after maximum concessions from America."

Rajabali Mazrouei, a former reformist lawmaker and political analyst, said Iran would be unable to close the strait.

"Khamenei's comments should be assessed as part of usual exchange harsh of rhetoric between Iran and the United States," he said. "Iran is in a position to temporarily disrupt oil shipments from the region but it will not be in a position to close the Strait of Hormuz permanently."

Khamenei's remarks appeared to reflect a deep concern for his country's future energy supplies. Despite its huge oil reserves, Iran already must import a large portion of the gasoline and diesel it needs because domestic refinery capacity is insufficient.

"That a country has no right to achieve proficiency in nuclear technology means it has to beg a few Western and European countries for energy in the next 20 years," he said. "Which honest leader is ready to accept this?"

After months of threats and counter-threats, Washington said last week it was prepared to join talks with Iran if it stopped enriching uranium, which can produce fuel for electricity-generating reactors or, if sufficiently processed, the fissile core for a warhead.

In conjunction with the U.S. offer, the five permanent U.N. Security Council members and Germany drew up a fresh economic incentive package for Iran last week but made it conditional on an end to enrichment. Iran could face sanctions if it declines the package.

Ahmadinejad said the Iranians would study the offer carefully but rejected preconditions.

Rice said the offer is not open-ended.

"I'm not one for timelines and specific schedules, but I think it's fair to say that we really do have to have this settled over a matter of weeks, not months," she said.

Javier Solana, the chief EU diplomat, was expected in Tehran soon to deliver the new incentive package.