Tension is rising sharply in relations between the United States and Iran, with three issues pointing to a possible crisis between the two countries after years of stalemate.
Washington has accused Tehran of giving sanctuary to operatives from the al-Qaeda movement.
Riyadh attack: US officials suspect an Iranian connection
It has also repeated its accusation that Iran is pursuing plans to produce nuclear weapons.
The two countries are at odds over Iraq as well, where the US is trying to minimise Iranian political influence as it tries to put together some kind of interim Iraqi administration.
Tehran has strongly denied all of Washington's charges.
It suspects that hardliners in the US administration are trying to pave the way for moves to undermine or change Iran's Islamic system.
What many officials and ordinary people in Iran are asking is whether the US has put Iran next on its list for hostile attention after Afghanistan and Iraq.
It certainly looks to many as though Washington is starting to orchestrate an anti-Iranian campaign.
There is no doubting the antipathy towards Iran felt by the hardline conservatives who appear to be in the ascendant in Washington.
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But events also seem to be playing in their favour.
US officials are apparently convinced that this month's multiple suicide bomb attacks in Riyadh, Saudi Arabia, were carried out in co-ordination with al-Qaeda leaders located inside Iran, including Egyptian-born Saif al-Adel. He is believed to be one of the movement's leading military planners.
Other Western diplomats in Tehran share the belief that senior al-Qaeda figures are in Iran, though they are unsure whether the Iranian authorities are condoning their presence, or whether they may not even know about them.
Iranian officials insist that they have detained and expelled more than 500 al-Qaeda suspects over the past year.
They say they are combating al-Qaeda now as they did long before the US turned against the radical Islamic movement.
But they admit that al-Qaeda suspects could be in the country without Iranian authorities being aware of it.
The accusations have apparently led to the suspension of discreet American-Iranian contacts that were being held in Geneva to try to iron out differences over Iraq.
US officials on Thursday failed to attend a meeting of the "six plus two" group - comprising the US, Russia and the six neighbours of Afghanistan - which has acted as a venue for official Iranian-American exchanges.
"The Iranians don't seem to have taken on board quite how strongly the Americans feel over the al-Qaeda issue," said one Western diplomat in Tehran.
There is wider international pressure on the nuclear issue. The G8 group of eight industrial nations on Friday joined the US in calling on Iran to sign an additional protocol that would allow tougher inspections to clarify Iran's nuclear intentions.
The G8 foreign ministers said that Iran's nuclear programme was "a cause of concern".
An inspection by the International Atomic Energy Agency of an Iranian facility at Natanz in February showed that Iran's uranium enrichment programme was much more advanced than the IAEA had imagined.
Clearly, there is no prospect of an imminent American attack on Iran at present.
But with all these issues brewing, an eventual confrontation is something that obviously cannot be totally ruled out.
Iranian officials say the leadership's policy is to avoid offering anything that might be regarded as a provocation.
At the same time, there is a conviction here that if the Americans want to attack Iran they will do it anyway. So there is much emphasis here on preparing for any such eventuality, with military and political leaders making almost daily declarations of military preparedness.
"If Bush is re-elected, the Americans will certainly put pressure on us, economically and politically, perhaps even striking at military targets," said one senior Iranian official.
"So the Islamic Republic is taking it very seriously, and making military and security preparations for defence, to face a new situation," he added.