By Paul Reynolds
World affairs correspondent, BBC News website
Iran's nuclear operations as Bushehr have raised concerns in the West
US President Barack Obama has reportedly made a potentially significant offer to Russia about defence - but will it change anything?
He has suggested that the US missile defence system in Eastern Europe would be halted if Russia used its influence to stop Iran from developing long-range missiles.
The idea is that if Iran did not pose a threat to the United States, then missile defence would not be necessary.
The US has the agreement of Poland and the Czech Republic to station anti-missile missiles and a radar station in their territories, which Russia has objected to.
The proposal, in a letter from Mr Obama to President Dmitry Medvedev, was first reported by the Russian newspaper Kommersant. Further details were then published by the New York Times and the Washington Post.
However, Mr Medvedev moved to quash the reports on Tuesday, insisting that although Russia was ready to co-operate on Iran, he had received no offer of "trade-offs" from his US counterpart.
"If we are to speak about some sort of exchange, the question has not been presented in such a way, because it would not be productive," he told reporters in Madrid.
Whether the US approach is a realistic proposition or a way of trying to entice Russia into applying more pressure on Iran over its missile and uranium enrichment activities remains to be seen.
But Iran is unlikely to give up its missile development programme easily, if at all.
Its ballistic missile development as well as its enrichment are already under Security Council sanctions.
I told the Russians a year ago that if there no were Iranian missile programme, there would be no need for the missile sites
US Defence Secretary
These might have slowed things down but certainly have not stopped them.
So, how Russia could intervene decisively is not clear. And the offer does seem to hinge not on Russian good intentions, but on results.
Iran's Shahab-3 rocket has a range of 2,000km (1,200 miles). This could reach Israel but not the US.
According to a report for the Congressional Research Service in February, Iran has no intercontinental ballistic missile (ICBM), defined by the US as one with a range of over 5,500km.
The report says there is "no universal agreement within the US intelligence community on the issue of an Iranian ICBM."
Some say, the report states, that Iran could test one by 2010 or 2015; others that there is a "less than even chance for such a test before 2015."
This offer, though, is evidence of the new thinking in Washington and of a desire both to resolve the Iranian problem, which could burst into a major crisis in due course, and to improve relations with Moscow.
It goes hand in hand with an American attempt to extend a hand to Iran as well.
US-Russian relations have been less confrontational since Mr Obama became president
US and Russian leaders have been less confrontational since the Obama administration came into office.
Now that goodwill has to be turned into the nitty-gritty of policy.
The US Under-Secretary of State, William Burns, who delivered the letter, told the Russian news agency Interfax: "If through strong diplomacy with Russia and our other partners we can reduce or eliminate [the] threat, it obviously shapes the way in which we look at missile defence."
The US Defence Secretary Robert Gates also said recently: "I told the Russians a year ago that if there no were Iranian missile programme, there would be no need for the missile sites."
And Russia's President Dmitry Medvedev said last weekend: "We have already received... signals from our American colleagues. I expect that these signals will turn into concrete proposals."
The offer will be discussed by the US Secretary of State Hillary Clinton and the Russian Foreign Minister Sergei Lavrov at their first meeting in Geneva on Friday.
That will not decide the issue which will be further explored in the first meeting between presidents Obama and Medvedev on 2 April in London, where they will both be attending the G20 economic meeting.
The offer comes as the International Atomic Energy Agency, which monitors Iran's enrichment development on the ground, is reporting that Iran has accumulated more than 1,000kg of low-enriched uranium.
If enriched to a much higher degree, this would be enough to make a nuclear device, according to the Chairman of the US Joint Chiefs of Staff, Admiral Michael Mullen.
However, Mr Gates himself said of Iran: "They're not close to a weapon at this point, and so there is some time."
Iran has said it has no intention of making a nuclear bomb and is enriching for nuclear power only.
To build a bomb, it would have to expel the IAEA inspectors, enrich its uranium to the higher level needed and leave the Nuclear Non-Proliferation Treaty, under which it is committed not to acquire nuclear weapons.
It would also need to be able to build a warhead and place it on a delivery system such as a missile.
All this would declare its intentions so openly that an attack on its nuclear plants by Israel and/or the United States, or even a wider coalition, would follow almost inevitably.