U.S. military action against Iran remains an option even as the United States pursues diplomacy and sanctions to halt the country's nuclear program, the Pentagon said on Wednesday.

"We are not taking any options off the table as we pursue the pressure and engagement tracks," Pentagon Press Secretary Geoff Morrell said. "The president always has at his disposal a full array of options, including use of the military ... It is clearly not our preferred course of action but it has never been, nor is it now, off the table."

Morrell was responding to reported comments by a top U.S. defense official who was quoted in Singapore as saying a strike on Iran was off the table in the near term.

Undersecretary of Defense for Policy Michele Flournoy said earlier Wednesday that the U.S. has ruled out a military strike against Iran's nuclear program any time soon, hoping instead negotiations and United Nations sanctions will prevent the Middle East nation from developing nuclear weapons.

"Military force is an option of last resort," Flournoy said during a press briefing in Singapore. "It's off the table in the near term."

The U.S. and its allies fear Tehran is using its nuclear program to build arms. Iran denies the charges, and says its program only aims to generate electricity.

"Right now the focus is a combination of engagement and pressure in the form of sanctions," Flournoy said. "We have not seen Iran engage productively in response."

Iran has rejected a 2009 UN-backed plan that offered nuclear fuel rods to Tehran in exchange for Iran's stock of lower-level enriched uranium. The swap would curb Tehran's capacity to make a nuclear bomb.

Iran has proposed variations on the deal, and Foreign Minister Manouchehr Mottaki said Tuesday that a fuel agreement could be a chance to boost trust with the West.

Earlier this week, he said Iran wants direct talks about the deal with all the U.N. Security Council members, except one with which it would have indirect talks - a reference to the United States, which with Tehran has no relations.

The U.S. is lobbying heavily in the Security Council for sanctions against Iran over its nuclear program.

Earlier Wednesday, Iran's supreme leader denounced U.S. "nuclear threats" against the Islamic Republic, and its elite military force said it would stage war games in a waterway crucial for global oil supplies.

The Revolutionary Guards' exercises in the Gulf and the Strait of Hormuz this week take place at a time of rising tension between Iran and the West, which fears Tehran's nuclear program is aimed at developing bombs. Iran denies the charge.

Iran has also reacted angrily to what is sees as U.S. President Barack Obama's threat to attack it with nuclear arms.

Obama made clear this month that Iran and North Korea were excluded from new limits on the use of U.S. atomic weapons -something Tehran interpreted as a threat from a long-standing adversary.

"The international community should not let Obama get away with nuclear threats," Supreme Leader Ayatollah Ali Khamenei said on Wednesday.

"We will not allow America to renew its hellish dominance over Iran by using such threats," he told a gathering of Iranian nurses, the semi-official Fars News Agency reported. Iran was a close U.S. ally before its 1979 Islamic revolution.

Brigadier General Hossein Salami, also quoted by Fars, said three days of maneuvers would start on Thursday and would show the Guards' naval strength.

"Maintaining security in the Persian Gulf and the Strait of Hormuz, as the world's key economic and energy routes, is the main goal of the war games," he said. "This war game is not a threat for any friendly countries."

Naval, air and ground forces from the Guards would take part, Fars said. The Islamic Republic's armed forces often hold drills in an apparent bid to show their readiness to deter any military action by Israel or the United States, its arch foes.

Nicole Stracke, a researcher at the Gulf Research Center in Dubai, said that with the "current threat to Iran increasing" the Guards were showing their capability and strength.

"The Revolutionary Guard is sending a message that we are ready and able to counter the threat," Stracke said in an e-mail to Reuters. But she added the force regularly held such drills and they were unlikely to increase regional tension.

Washington is pushing for a fourth round of UN sanctions on Tehran over its refusal to halt sensitive nuclear activities as demanded by the U.N. Security Council, including moves against members of the Guards.

Israel, widely believed to have the Middle East's only atomic arsenal, has described Iran's nuclear program as a threat to its existence. Although it says it wants a diplomatic solution, Washington has also not ruled out military action.

Iran, a predominantly Shi'ite Muslim state, has said it would respond to any attack by targeting U.S. interests in the region and Israel, as well as closing the Strait of Hormuz. About 40 percent of the world's traded oil leaves the Gulf region through the strategic narrows.

Salami made no reference to this in his comments, stressing Iran's "efficient and constructive role" for Gulf security.

"Peace and friendship, security, tranquility and mutual trust are the messages of this war game for neighboring countries in the Persian Gulf region," the general added.

Sunni-led Arab countries in the Gulf are concerned about spreading Iranian influence in the region and also share Western fears about Tehran's nuclear ambitions.

Cliff Kupchan, a director of Euroasia Group, said in a note on Wednesday that he still believed that Israel was unlikely to strike Iran, but "the risk will grow as prospects for successful sanctions diminish". China and Russia, veto-wielding Security Council members, are reluctant to back tough sanctions on Iran.