Though Washington says it is committed to diplomacy, its more muscular tone, and Iran's refusal to halt urananium enrichment, mean a military confrontation cannot be ruled out, they said.

"Everything is within the realm of possibility at the moment, this is what makes the situation so harrowing," said Karim Sadjadpour, an Iran specialist with the International Crisis Group.

"In 2007 we could talk about a situation where Iran is bombed, or the US and Iran have made amends."

The administration has noticeably toughened its language in the last week at a crucial point in the global bid to convince Tehran to suspend uranium enrichment, in return for a package of incentives.

Washington says Iran has stalled enough, and should face sanctions. Tehran denies it is making the bomb.

President George W. Bush said Iran's leaders were "tyrants" and must not be allowed "tools of mass murder."

Secretary of State Condoleezza Rice said the US drive for sanctions tested United Nations credibility -- an argument familiar from the febrile period before the Iraq war.

The top US arms control official Robert Joseph said the notion of a nuclear-armed Iran was "intolerable" while a new US Anti-Terrorism Strategy warned of "the potential WMD-terrorism nexus that emanates from Tehran."

Administration tub-thumping does not by itself signal a slide into military action -- though it set Washington's community of foreign policy think-tanks abuzz.

In a sign of the complexity of the situation, Bush moderated his tone in a Wall Street Journal interview Friday, saying he had signed off on a US visit by Iran's ex-president Mohammad Khatami as he wanted to learn more about Iran.

Some analysts fear raising the stakes with Iran risks locking each side into dangerous positions, while Chinese and Russian reluctance to punish Tehran, and Iran's own deft political tactics offer ingredients for a deepening crisis.

Gregory Gause of the University of Vermont, said that while US rhetoric must be viewed in the light of looming congressional elections, it could not be dismissed as purely tactical.

"This is not an administration that makes idle threats, it fits perfectly into the Bush doctrine ... I see no basic change in the way they view the post 9/11 world," he said, referring to the notion that the United States must confront threats preemptively -- by military action if necessary.

Some of Bush's arguments seem patented after those used on Iraq.

"When we saw the damage the terrorists inflicted on 9/11, our thoughts quickly turned to the devastation that could have been caused with weapons of mass destruction," he said last week.

"Iran must end its support of terror, it must stop defying its international obligations and it must not obtain a nuclear weapon"

Some analysts think Washington may be trying to jolt reluctant China and Russia into agreeing to sanctions, and to stiffen European resolve, as EU foreign policy chief Javier Solana holds last ditch talks with Iran in Vienna.

The White House may also be sending a coded warning to Iran -- perhaps to debunk perceptions a military strike is unlikely because it would expose US troops in Iraq, and has no guarantee of success.

"Don't think just because Iraq is maybe in chaos, we have our hands full in Lebanon, and oil is at 75 dollars a barrel that we have exhausted our capabilities," Sadjadpour said, paraphrasing a possible US position.

But ratcheting up rhetoric could leave the United States with a dilemma if sanctions don't halt Iran's nuclear program.

"At some point, the president faces a binary choice between standing by and doing nothing as Iran continues to cross these thresholds, or ordering unilateral military action in an effort to try and delay that development," said Flynt Leverett, a former Bush national security council aide.

"If he is faced with that binary choice, I think his chances of choosing a strike are not trivial," said Leverett, now with the New America Foundation think-tank.

Gause believes a military strike is possible given the two sides seem on collision course.

"I don't think the Iranians are going to give up the enrichment cycle," he said.

Sadjadpour said that Iran and the United States were more likely to settle into a prolonged face-off as "the cost of escalation is so high."

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