By Paul Reynolds
World Affairs correspondent, BBC News website
American rhetoric about Iran has been stepped up because it wants to pressure the Iranian government to give up any ambition to build a nuclear bomb.
Condoleezza Rice: US attack on Iran "not on agenda at this point"
Talks are currently going on between Iran and three European countries - France, Germany and the UK - about Iran's nuclear programme.
The United States wants it to be known that there will be consequences in the event of failure.
These talks are likely to reach a conclusion one way or another in the summer. So there is still a window open for diplomacy.
Which is why the new US Secretary of State Condoleezza Rice said at a news conference in London that a US attack on Iran was "not on the agenda at this point."
That means, however, that it might get onto the agenda at a later point.
When political leaders do not rule something out, it is sensible to keep a look out.
Secretary Rice's comments follow the strong words used about Iran by President Bush in his State of the Union address when he declared: "And to the Iranian people, I say tonight: As you stand for your own liberty, America stands with you."
The United States has not declared hostilities against Iran. But it has declared hostility.
So why Iran and why now?
The background is the deep antipathy between the United States and Iran since the Shah was overthrown in 1979 and an Islamic republic was set up. This colours all their relations.
The United States regards Iran as troublemaker in the region, helping Hezbollah fight Israel from Lebanon and helping Hamas in the Palestinian territories. Iran regards the United States as an evil empire.
A diplomatic campaign against Iran's rulers also fits in with the agenda of the Bush administration to foster the spread of what Mr Bush referred to as the "guiding ideal of liberty for all".
Cynics might argue that it also helps to divert opinion from the problems in Iraq.
But the immediate issue is Iran's nuclear programme.
In 2002, Iran was forced to admit that for 18 years it had been secretly developing the ability to enrich uranium, the first step in building a nuclear device.
As a member of the Nuclear Non-Proliferation Treaty, Iran is not allowed to make a nuclear bomb but it can develop a civilian nuclear power programme.
It has argued that it needs to learn how to enrich so it can produce fuel for its nuclear power plant, which is still under construction.
But because of its secret work, the West does not trust it anymore.
Western countries do not want Iran to have any enrichment capability because the technology used to enrich uranium to a standard needed for nuclear power can also be used to enrich it further to a standard needed for a nuclear explosion.
The United States failed last year to get Iran reported to the UN Security Council to face possible sanctions.
The three European countries are now negotiating with Iran to turn a freeze on its enrichment activities into a cessation.
Iran says its nuclear regime is peaceful
In return Iran will be offered nuclear fuel supplies, improved trade arrangements and possibly security guarantees.
Iran on the other hand says that enrichment must go ahead though it is willing to accept strict inspections, or "objective guarantees" as they are called in the diplomatic jargon.
If this effort collapses, then there will be further talk about taking Iran to the Security Council to face sanctions.
All this has left the Bush administration deeply sceptical about Iran's intentions and it has not taken part in the negotiations itself.
Instead it is now issuing threats and warnings from the sidelines.
This is a well-known diplomatic tactic like the good and bad cop routine. But in this case there is something else and that something else is Israel.
Lieutenant General Shaul Mofaz, the Israel Defence Minister, said in London recently: "Iran is the main threat to Israel in the long run."
Like Ms Rice, he too said that diplomacy was the route for the moment though, like her, he also refused to rule anything else out later on.
It is possible that if the talks fail and Iran presses ahead with enrichment, then Israel would consider mounting an air attack.
The US Vice President Dick Cheney mentioned the possibility the other day.
However, the military options are not good given that Iran's nuclear facilities are spread out widely.
According to Dr Gary Samore, of the International Institute for Strategic Studies in London, there is an optimistic and a pessimistic scenario.
"The optimistic view is that Iran does not want a confrontation with the Security Council and might rest on its laurels. It has probably not decided yet.
"The pessimistic view is that Iran is simply going through the motions with the Europeans and that these talks are doomed."