By Barbara Plett
BBC UN correspondent, New York
Washington and European countries believe Iran wants to build the bomb
Statements out of Washington and other Western capitals suggest a shift from engagement to confrontation with Iran over its nuclear programme.
"What we are going to be working on over the next several weeks is developing a significant regime of sanctions that will indicate to them how isolated they are from the international community as a whole," President Barack Obama told journalists earlier this week.
The president sounded not unlike his predecessor George W Bush, who worked for years to contain Iran, a sign that Mr Obama's policy to engage with the Islamic Republic has failed.
The question now is whether a new round of sanctions would be any more successful than those imposed under Mr Bush's watch.
For the last six months the US has led the five permanent members of the UN Security Council and Germany (the so-called P5+1) in negotiations trying to persuade Iran to suspend its enrichment of uranium, as demanded by UN resolutions.
Washington and the European countries believe Iran wants to build the bomb. Tehran says its programme is for peaceful purposes only.
Last year the P5+1 offered Iran a compromise proposal in which it could send most of its low-grade nuclear fuel abroad for further enrichment.
Iran accepted, hedged, then rejected the proposal. This week it announced it would make the higher grade fuel itself.
The confusion has hardened the US view that sanctions are now the only way forward.
"We've bent over backwards to say to the Islamic Republic of Iran that we are willing to have a constructive conversation about how far they can align themselves with international norms and rules," said Mr Obama.
"They have made their choice so far."
The Americans are circulating a catalogue of suggested sanctions amongst Western allies.
A key measure is a greater focus on targeting Iran's Islamic Revolutionary Guard Corps - overseers of the nuclear programme - with travel bans and asset freezes.
The stated goal remains to get Iran back to the negotiating table.
"The measures would be designed to put maximum pressure on decision makers," said a Western diplomat, "to show that there's an economic and political cost that's too high, but also to show there's a way out."
The highest cost would come through sanctions on the energy sector, something supported by both France and Britain, say diplomats. To have teeth they would require the backing of Russia and China, but there's little chance of that.
True, Moscow has signalled that it's dropped traditional opposition to sanctions, influenced in particular by Iran's rejection of the nuclear fuel deal, which would have sent uranium to Russia for enrichment.
But Russia is coy about substance. And China, which has large trade with and investment in Iran's oil and gas sector, remains opposed.
The US and its allies will continue trying to convince Beijing to change its mind. They would prefer to avoid a Chinese abstention on a Security Council resolution, arguing that even weak sanctions would be better than breaking the unity of the P5+1.
UN sanctions have failed to induce Iran to change its nuclear policy.
"If we keep together, we do have influence," said one Western diplomat. "When it comes to influencing Iran, the unity of the international community really counts."
A UN resolution would also lay the groundwork for tougher sanctions from Europe and America.
"After (the UN resolution) you have the substantial sanctions which are bilateral," said another diplomat. "EU regulations, for instance, have always been more stringent than the UN sanctions. So it's a political basis on which we want to increase the political pressure on Iran."
But so far three rounds of Security Council sanctions as well as bilateral measures have failed to induce Iran to change its nuclear policy.
That's because they haven't been harsh enough, says Israeli Prime Minister Benjamin Netanyahu. He calls for "crippling" not "moderate or watered-down sanctions."
Israel - which has an undeclared arsenal of nuclear weapons - has not ruled out a military strike on Iran, a scenario the P5+1 are desperate to avoid.
Others say the purpose of the sanctions should shift - from changing Iran's policy to changing Iran's regime.
Behind this argument lies the belief that the government has been seriously weakened by internal political turmoil since the contested presidential elections last year.
Tough sanctions would help bring it to its knees, say the regime changers.
"In essence the Iranian regime must be given a choice, either the bomb or survival," said the Israeli Vice-Prime Minister Moshe Yaalon recently.
"Regime change in Tehran is the best non-proliferation policy," agreed Robert Kagan of the Carnegie Endowment for International Peace, writing in the Washington Post.
"President Obama has a once-in-a-generation opportunity in the next few months to make the world a dramatically safer place
by helping the Iranian people achieve a new form of government."
But some say the idea that the Iranian regime is about to implode is misguided, as has been Barack Obama's entire engagement policy.
According to this view, rather than focusing narrowly on nuclear issues, he should have advocated a strategic relationship on a range of issues, treating Iran as a regional power dealing with legitimate security issues rather than a rogue state.
"Instead of pushing the falsehood that sanctions will give America leverage in Iranian decision making - a strategy that will end either in frustration or war - the administration should seek a strategic alignment with Iran as thoroughgoing as that effected by Nixon with China," writes Flynt Leverett of the New America Foundation in the New York Times.
"On that basis America and Iran would forge a comprehensive framework for security as well as economic co-operation, something that Washington has never allowed the P5+1 group to propose."
It's not clear whether the Iranian regime, facing both external opposition and internal division, would be capable of responding to such a proposal.
But neither is it clear how more sanctions would change Iran's behaviour without risking increased regional tension or even conflict.