The White House welcomed as a positive step Iran's decision to suspend its uranium enrichment programme - but it was a very guarded welcome.
The Bush administration is reluctant to praise the actions of a country it believes is a member of the "Axis of Evil" and the administration will be deeply sceptical about whether Iran is really prepared to renounce its nuclear ambitions.
The White House spokesman, Scott McClellan, stressed that Iran should now sign a formal agreement - the so-called Additional Protocol - allowing more intrusive inspections.
Nevertheless, speaking in Singapore on the president's tour of East Asia, Mr McClellan thanked the three European countries who had secured the agreement.
"We have been in close contact with the Europeans all along," said Mr McClellan.
"So we very much welcome today's efforts by the Europeans to obtain a commitment from Iran.
"What is essential now is that Iran needs to fully comply by signing and implementing the additional protocol, co-operating fully with the IAEA, and taking steps to end its uranium enrichment and reprocessing activities."
Hardliners in Washington are sceptical of the whole principle of arms control agreements but for the moment it is difficult for them to criticise Iran for doing exactly what it was asked to do.
"They are not going to praise Iran, but there is not a lot for them to complain about," explained Terry Taylor of the International Institute for Strategic Studies (IISS).
Washington's concern, he explained, was that Europe may have just "ticked the box" - done enough to take the crisis off the boil without actually solving the problem.
The US believes there is no justification for Iran to have any kind of civilian nuclear programme, bearing in mind its massive oil reserves.
The Europeans do seem to have conceded that principle.
But Washington is also trying to prevent Iran from driving yet another wedge between the transatlantic allies.
The Europeans believe in engaging Iran. The US believes in a policy of isolation.
"Iran is responding and I think it calls for the US to at least rethink its isolationist policy for Iran," argues David Albright, a former UN weapons inspector who now heads the Institute for Science and International Security (ISIS) in Washington.
"Europe should get more from Iran, but it's enough for the US to start rethinking its policy on Iran, which is based on calling them names and isolating them."
Memories of 1979
As with so much in the Bush administration, policy towards Iran is bitterly fought over.
At the state department, Colin Powell would like to engage with the country and strengthen the position of moderates within the Iranian government.
Hardliners within the Bush administration point out that there has been precious little to show for that approach.
Sentiment towards Iran is still coloured by memories of the seizure of the American embassy in Tehran in 1979, and the long subsequent hostage crisis.
The US does not easily forgive such humiliations.
The Bush administration believes the invasion of Iraq changed the international landscape, putting what it believes are "rogue states" such as Iran and North Korea on notice.
But there are very few advocates in Washington of military action against either country.
So Iran's latest move could be the beginning of a very slight thaw in relations with the US.