By Paul Reynolds
World affairs correspondent, BBC News website
The aircraft carrier USS Stennis adds to US power in the Gulf
The tension over Iran's nuclear programme is increasing, with its failure to comply with a Security Council deadline to suspend uranium enrichment.
The UN's nuclear agency, the International Atomic Energy Agency (IAEA), reported that the deadline of 21 February had passed with no Iranian action.
"Iran has not suspended its enrichment-related activities," the IAEA stated.
(Update 26 February: adding to the tense atmosphere is an article in the New Yorker magazine from Seymour Hersh who says that the Pentagon has been told to draw up a bombing campaign against Iran that could be launched at 24 hours' notice. Hersh also says that the United States is undertaking clandestine operations against Iran and the Lebanese group Hezbollah as part of its wider Middle East strategy of gathering support among Sunni Arab countries to try to limit Iran's influence.
The Pentagon responded by saying that the "The United States is not planning to go to war with Iran. To suggest anything to the contrary is simply wrong, misleading and mischievous.")
The council had given Iran 60 days from the passage of resolution 1737 on 23 December to suspend all enrichment activities and also work on heavy water projects. The latter could give Iran a supply of plutonium, an alternative source to enriched uranium for a nuclear explosion.
Resolution 1737 imposed economic sanctions on Iran, aimed at stopping the transfer of technology to its nuclear and missile industries. It also said that if there was no compliance after the 60-day deadline, further sanctions would be considered.
The resolution supported an offer from Western countries to help Iran develop civilian nuclear power -- but it had to suspend enrichment as a condition for any talks.
The IAEA finding came as no surprise as the Iranian President Mahmoud Ahmadinejad says that there can be no pre-conditions for talks. Indeed, he has proposed that Western governments suspend enrichment themselves before any talks. The president has also declared that Iran's nuclear programme has "no brake and no reverse gear".
The issue now is whether the council will follow up on its threat to impose more sanctions.
The resolution is clear that these would have to be economic in nature and that there will have to be a new decision by the council as a whole to impose them.
The United States is already calling for such measures. However getting Russian and Chinese agreement is likely to be a slow process.
The head of the IAEA, Mohamed ElBaradei, is suggesting a mutual freeze - by Iran on its nuclear development and by the Security Council on sanctions. No freeze is likely.
Mr ElBaradei told the Financial Times this week that Iran might be five or 10 years away from developing a nuclear bomb. He warned against "hype" over Iran's nuclear activities.
At the same time, the BBC has reported that the United States has drawn up plans for an attack on Iran to cover two contingencies - the confirmed development of nuclear weapons by Iran, or backing by Iran for a major attack on US troops in Iraq.
The first contingency is full of uncertainties. Iran says it is simply exercising its right to provide fuel to make nuclear energy and that it has no intention of building a bomb.
The problem is that the same technology used to make fuel for nuclear power can then be developed to make fuel for a nuclear explosion.
The US continues to apply pressure on Iran and has sent a second aircraft carrier battle group into the Gulf region.
On a visit to Australia, Vice-President Dick Cheney said it would be "a serious mistake" to allow Iran to become a nuclear-armed country. "All options" were on the table, he said.
A new element emerging over the last couple of weeks is the linkage the US is making between Iran and events inside Iraq. It has publicised its contention that Iran is behind sophisticated technology that is being used by some Shia groups against US and British forces in Iraq.
The timing of this claim, rejected by Iran, is significant, because it ties in with the expiry of the Security Council demand on 21 February. It adds a new component to the equation.
The US can now claim a casus belli if there is a major attack on US forces in Iraq that can be linked to Iran. Such linkage of course is not easy to prove, and even the evidence that the US has produced so far has been challenged.
The legality of any attack against Iran will be hard to establish, to say the least, without clear evidence, especially as the evidence against Iraq proved unreliable.
Claims by a US official in Baghdad that the Iranian leadership knew about the provision of technology to Iraqi Shias was undermined by a subsequent statement from the Chairman of the Joint Chiefs of Staff, General Peter Pace. "We know that the explosively formed projectiles are manufactured in Iran. What I would not say is that the Iranian government, per se, knows about this," he said on the Voice of America.
All this makes for an extremely delicate and dangerous period ahead.
But there is a diplomatic effort at play here as well.
Washington hopes that its pressure will trigger not necessarily a war but a debate inside Iran that will either lead to a change of policy (maybe through a change in government) or a much slower and more cautious Iranian approach.
It is also not clear that within the Bush administration these days there is total support for any attack on Iran. The influence of Secretary of State Condoleezza Rice seems to be growing at the expense of Vice-President Cheney.
We have seen the US entering negotiations over North Korea, leading to an interim agreement under which the North's claimed nuclear weapon is being left to one side.