Today is deadline day for Iran to suspend the enrichment of uranium, as per a United Nations Security Council resolution.
Since that's probably not going to happen, the BBC's Paul Reynolds tries to analyze what will happen next.
The UN's nuclear agency, the International Atomic Energy Agency (IAEA), is to report this week on Iranian compliance with the Security Council demand.
The IAEA head Mohamed ElBaradei warned against 'hype' over Iran's nuclear abilities
The head of the IAEA, Mohamed ElBaradei, is calling for 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.
Meanwhile, the US continues to apply pressure on Iran and is expected to move a second aircraft carrier battle group into the Gulf region soon.
US Vice-President Dick Cheney said in Newsweek magazine that American allies in the region "want us to have a major presence there" and that the carriers would send " a strong signal" that the US would "work with friends and allies to oppose the Iranian threat".
Iraq linkage
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 US claims Iran is supplying weapons to Iraq insurgentsThe timing of this claim, rejected by Iran, is significant, because it ties in with expiry of the Security Council demand on 21 February. It adds a new component into 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.
But 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.
Danger
All this makes for an extremely delicate and dangerous period ahead.
It does not mean that a US attack on Iran is imminent. The BBC information is that the US has chosen targets in Iran and has considered two scenarios for an attack.
The targets include not only Iranian nuclear sites but Iranian missile sites and other major military infrastructure.
This would be in line with US doctrine that, in a conflict, an attack has to cover the range of military targets. This happened in the two Gulf wars and Israel adopted similar tactics in its attacks on Hezbollah last year.
But it is not an either-or situation.
