The West is talking tough but has few options to increase the pressure on Iran following a stark new report by the UN's nuclear watchdog that makes it clear Tehran conducted extensive research into building a nuclear weapon.
Israel is pressing for "crippling" sanctions that would target Iran's Central Bank and oil exports, but US officials are ruling out both measures because they would further damage an already reeling global economy. They would also hurt ordinary Iranians.
The administration of US President Barack Obama is playing down the military option for the time being. Bombing Iran could spark a regional conflict, send oil prices spiralling, unify Iran's ruling hardliners and convince Tehran of the need to weaponise its nuclear programme.
Even Israel, the region's sole nuclear-armed power, which has a long-standing doctrine to strike pre-emptively at any nation it perceives to be an existential threat, has toned down its rhetoric of recent days. "War is not a picnic … We don't want a war," Israel's defence minister, Ehud Barak, said shortly before the UN report was released on Tuesday.
Sir Richard Dalton, Britain's former ambassador in Tehran, said: "I just don't know what the US and the Europeans can do [to increase economic pressure on Iran] without the backing of Russia and China."
Both big powers are veto-wielding, permanent members of the UN Security Council, which have strong trade relations with Iran. China, in particular, needs Iran to help meet its insatiable appetite for oil.
Russia and China supported four earlier sets of UN sanctions but only after ensuring their effect was diluted. Rather than new sanctions, Moscow and Beijing want to revive negotiations between Iran and the five permanent members of the Security Council and Germany, although years of fitful talks have led nowhere.
Moscow, this summer, proposed easing existing sanctions in return for greater transparency from Iran on its nuclear programme. Tuesday's UN report did not estimate how long it would take for Iran to develop a nuclear weapon but western diplomats believe Tehran could still be up to three years away, leaving time for a diplomatic solution.
The UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), could give Iran a notice period in which to answer its concerns, after which it might be more difficult for Moscow and Beijing to block a referral to the UN Security Council.
But Sir Richard said in an interview that the talks logjam can only be broken if the six world powers "start negotiating seriously on the terms for Iran to exercise its right to enrich uranium rather than appearing simply to deny them that right".
Any concessions by the US administration, however, would probably spur a backlash by Mr Obama's Republican opponents. Some of them are calling for military action and accuse the president of being soft on Iran, which insists its nuclear programme is designed to generate electricity.
Scott Lucas, an expert on relations between the US and Iran at Birmingham University in England, said US domestic politics appeared to be driving Washington's policy on Tehran. "The Iranians are already under a great deal of economic pressure because of the sanctions. It's hard to see what the US can do more than try to tighten existing measures," he said in an interview.
That, and persuading its western allies to impose additional bilateral sanctions, is seemingly Washington's strategy at the moment.
Some western officials say it is time for Gulf Arab nations, which in private are deeply concerned about Iran's nuclear programme, to "break surface" and demand that Russia and China back new UN sanctions.
Saudi Arabia, in particular, has leverage: the kingdom is one of China's top crude oil suppliers and Chinese companies are deeply involved in Saudi infrastructure projects. Beijing and Moscow must also be aware that blocking attempts for further sanctions could bolster the case for military action as a last resort.
In the meantime, the US is expected to impose further sanctions on Iranian commercial banks, front companies and individuals suspected of links to Tehran's nuclear programme, while trying to close loopholes in existing sanctions.
Tuesday's report by the IAEA, however, made clear that years of sanctions have failed to persuade Iran to reconsider its nuclear programme.
Israel's hardline foreign minister, Avigdor Lieberman, said that if the US does not spearhead a drive for "crippling" sanctions against Iran, it will mean Washington and the West "have accepted a nuclear Iran".