By Jonathan Marcus
BBC Diplomatic correspondent
Western powers suspect Iran's nuclear ambitions are not peaceful
The developing diplomatic row over Iran's nuclear ambitions has highlighted the question of consistency in US and Western efforts to halt the spread of nuclear weapons.
Close US ally Israel is widely believed to have an advanced nuclear arsenal which rarely, if ever, draws any criticism from Washington.
India is quite open about its nuclear weapons programme, but this has not stopped the Americans from proposing an ambitious programme of civil nuclear co-operation with the Indians.
So has strategic interest trumped consistency in the non-proliferation field?
Iran has frequently charged that it is being treated unfairly. It insists that its nuclear ambitions are solely for peaceful purposes.
Iranian experts say other countries have fully-fledged nuclear weapons programmes, but they do not incur Washington's wrath.
One of the great concerns is that Iran could follow North Korea's route, accepting the constraints of the Non-Proliferation Treaty for now - and then breaking out once its nuclear programme is sufficiently mature.
Iran has been far from forthcoming about much of its past nuclear history - and that is one reason why there is so much concern
However, North Korea shows some of the limitations of diplomacy in tackling such thorny non-proliferation issues.
But military options to halt its nuclear programme are almost unthinkable - just as with Iran.
The onus remains on the diplomats to find a way through this complex crisis which involves energy policy, security issues and basic nationalism.
So what of Iran's claim of there being double-standards, especially in Washington?
There is little doubt that both India and Israel have a nuclear weapons capability.
Both though maintain close ties with the Americans. Israel has a very close military relationship with Washington and the Bush administration seems to have thrown initial reservations about India's nuclear programme to one side and is now eager to step-up nuclear co-operation, at least in the civil field.
So what price consistency?
In stark diplomatic terms Israel and India are in a different category to Iran.
Neither India nor Israel, nor Pakistan for that matter - which is also thought to have a small nuclear arsenal - have signed the Non-Proliferation Treaty.
Thus they are not breaking their treaty obligations in pursuing a nuclear weapons programme. However, Iran has signed the Treaty and is bound by it.
Iran has been far from forthcoming about much of its past nuclear history - and that is one reason why there is so much concern.
But diplomacy is not just about observing treaties; it is about sending the right signals.
And many US arms control experts see the Bush administration's plans for civil nuclear co-operation with India as driving a coach and horses through the broader non-proliferation regime.
Viewed from Washington, consistency is not so much the issue as interests. Israel and India are key strategic allies of the United States.
They are democracies. Their arsenals are not seen as destabilising - in fact, it is quite the opposite.
And Iran, at least for the Americans, falls into a very different category.