* Obama pursues sanctions on Iran to curb nuclear programme
* Tehran vows to shrug off new measures as in past
* U.S. offer of dialogue gives way to punitive action
By Alistair Lyon, Special Correspondent
BEIRUT, April 12 (Reuters) - The United States looks short of Iran options. Even President Barack Obama acknowledges that new sanctions might fail, but has not laid out another plan.
Iran, which has had stormy ties with Washington since its 1979 revolution ousted the U.S.-backed shah, has sworn to defy any harsher measures over what it says is a peaceful nuclear programme. The West suspects Tehran is seeking atomic bombs.
"The broad sanctions track seems to be pursued more due to a sense of political necessity rather than confidence that it will resolve the nuclear problem," said Trita Parsi, director of the Washington-based National Iranian American Council.
"The argument now is that the pressure in the past wasn't strong enough and not multilateral enough. But that has been the argument every time new sanctions have been imposed on Iran, and every time the result remained the same," he said.
Iran appears determined not to blink. On Sunday it announced it had begun producing an advanced anti-aircraft missile system, two days after unveiling a faster centrifuge to enrich uranium.
Obama's offer last year to "engage" the Islamic Republic met a chilly response. It withered when the Iranian authorities crushed waves of street protests over a disputed June election that kept hardline President Mahmoud Ahmadinejad in power.
Strikingly, a president who last year stressed how America's soft power could help forge a new start in ties with the Islamic world, this month explicitly excluded Iran and North Korea from tighter limits on the possible use of U.S. nuclear weapons.
Obama, perhaps responding to domestic pressure not to appear weak on Iran, ruled out any nuclear attack on non-nuclear armed states -- apart from Iran and North Korea, which he said were not complying with the Nuclear Non-Proliferation Treaty.
Ahmadinejad, who often speaks on nuclear policy, even if the final word rests with Supreme Leader Ayatollah Ali Khamenei, rebuked Obama's move as the work of an "inexperienced amateur".
Some of Obama's critics say he plans to switch to containing a nuclear-armed Iran, an idea Pentagon chief Robert Gates sought to dispel on Sunday. He said Iran was not yet "nuclear capable" and denied it was inevitable that it would get the bomb.
PRESSING FOR SANCTIONS
Iran is not among the 47 nations at this week's Washington conference on how to keep "nuclear terrorists" from acquiring bomb-grade material. Obama will press his case for tougher sanctions against Tehran in side-meetings with foreign leaders.
He wants the U.N. Security Council to agree such measures within weeks, although veto-wielding powers Russia and particularly China may try to dilute them before consenting.
Three previous rounds of U.N. sanctions, along with unilateral U.S. and European measures, have discouraged foreign investment in Iran and inflicted extra burdens on the economy.
"However, it is not clear if economic costs alone will have a critical role in shaping the Iranian government's thinking on the nuclear programme," said RAND Corporation's Alireza Nader.
"The Revolutionary Guards' consolidation of power after the June 2009 election has marginalised pragmatic elements more mindful of the nuclear programme's long-term economic costs."
Nevertheless, Ahmadinejad's government is concerned about sanctions, Nader said, citing diplomatic efforts to sway Security Council members in an effort to avoid isolation.
Foreign Minister Manouchehr Mottaki said last week Iran was still ready for a nuclear fuel swap to allay tensions with the West, as long as the exchange took place on Iranian soil.
In October Iran agreed in principle to send low-enriched uranium abroad for more processing, only to change its mind.
Reviving talk of a fuel deal is aimed at helping China and Russia make the case for diplomacy, rather than punitive options, said Mahjoob Zweiri, a Qatar-based Iran analyst.
"The Iranians are saying: 'There's an opportunity, let's talk about it. We are not bad guys, we are flexible'."
Obama declared last week there was no guarantee that Iran would heed new sanctions, but said steady international pressure might eventually change its nuclear calculations.
WISHFUL THINKING?
That smacks of wishful thinking, said Paul Ingram, executive director of the British American Security Information Council, who argued that time was not on the side of those who expected Iran's bargaining hand to weaken as it slowly mastered enrichment technology and amassed low-enriched uranium stocks.
"New international sanctions won't be the 'crippling' ones sought by the West," he said. "There are likely to be plenty of sanctions-busting operations that reduce their impact."
Ingram said sanctions were likely to strengthen Iran's political resolve and foster self-sufficiency in its economy.
Iran's nuclear work is only one of many irritants that have poisoned U.S.-Iranian relations for the past three decades.
"The United States perceives Iran as a threat because of its foreign policy, its slogans, its behaviour in the region and its way of building relations with non-state actors," said Zweiri, referring to armed groups such as Hamas and Hezbollah.
The Islamic Republic's "real sins" were its identity and the nature of its birth, Ingram said. "Nuclear programmes are the icing on the cake that convert the threat from a regional to a global one in the minds of those who live by fear."
Israel certainly views Iran's nuclear ambitions as its gravest danger, even if U.S. pressure has induced it to tone down talk of military action to give diplomacy a chance.
But if sanctions fail, might Obama weigh military options?
"Appetite for military action in Washington is extremely low," said Parsi, author of a history of relations between the United States, Israel and Iran.
"However, there is a legitimate concern that if sanctions are considered a political necessity now, will military action be regarded as a political necessity in 2011, once the sanctions have been deemed a failure?"