By Paul Reynolds
World affairs correspondent, BBC News website
The possible timing of any Israeli attack on Iran remains uncertain
The warning by the senior US military commander Adm Mike Mullen that an attack on Iran would be "extremely stressful" for US forces must lessen the chances of the US taking part in any strike against Iran.
But the admiral, who is chairman of the joint chiefs of staff and who has just visited Israel, spoke of Israel's vulnerability to "very real threats".
So the possibility remains that Israel might undertake an operation against Iran by itself. Recent large-scale Israeli air force exercises have strengthened this possibility, according to military observers.
Nor does Adm Mullen's intervention resolve the ambiguity of the Bush administration's position that "all options" are on the table.
But his views do indicate that the body of US military opinion is that they have their hands full in Iraq and Afghanistan.
Administration at odds
Adm Mullen's opinion echoes what the then head of Central Command, Adm William Fallon, said last November, that an attack on Iran was not "in the offing".
Adm Fallon resigned in March amid reports that he was at odds with the administration over Iran.
Iran is not, openly at least, making highly enriched uranium suitable for a weapon, only low-enriched uranium useable as nuclear power fuel
Increasingly, the military option seems to be narrowing to an Israeli option.
While Adm Mullen did not diverge from the Bush administration's line that the military option remains for the US and also said that in his view Iran was working to develop nuclear weapons, he stressed that "the solution still lies in using... diplomatic, financial and international pressure".
Military opposition to an attack on Iran is bound to weigh heavily on President George W Bush but would not necessarily be the determining factor.
Whether President Bush would dissuade Israel from launching its own attack is not known.
Iran has warned that any attack would bring consequences, one of which could be an Iranian move to close the Straits of Hormuz, through which oil is transported from the Gulf. The effect on oil prices would be serious.
An Israeli cabinet minister and former chief of staff, Shaul Mofaz, has said that an attack on Iran is "unavoidable" if it "continues with its nuclear programme".
However, the timing of any attack remains uncertain.
A recent ABC News report suggested that Israel might act before two "red lines" are reached.
The first would be the production by Iran of enough highly-enriched uranium to make a nuclear bomb and the second would be its acquisition of a new Russian anti-aircraft system, the S-300.
However, Iran is not, openly at least, making highly enriched uranium suitable for a weapon, only low-enriched uranium useable as nuclear power fuel. (Update 4 July: the evidence for this comes from the 26 May 2008 report from the IAEA, released on 5 June. This states that "the results of the environmental samples... indicate that the [enrichment] plants have been operated as declared. The samples show low-enriched uranium... particles.")
(Further update 28 August: it has been pointed out that some people are sceptical of the claim by Iran that it is enriching only to low values. For example, the US National Intelligence Estimate stated: "We assess with moderate confidence that Iran probably would use covert facilities rather than its declared nuclear sites for the production of highly enriched uranium for a weapon."
And the former head of Israeli Military Intelligence Maj-Gen Aharon Ze'evi-Farkash told the Jerusalem Post that, recognising that the international community had discovered their covert programme (in 2002), the Iranians set about hiding its traces, but then resumed their activities.)
The International Atomic Energy Agency would probably spot any move to high enrichment at the inspected plants. But even given the lack of certainty over a secret programme, it is unclear exactly how and when this "red line" might be reached is unclear.
As for the S-300, it was only in December that Iran indicated that it would buy this very advanced anti-aircraft system. It has only recently taken possession of the Tor-MI and it could be many months before the S-300 is delivered.
Iran says that it has no intention of developing nuclear weapons and a US National Intelligence Estimate has concluded that it probably gave up a nuclear weapons programme in 2003.