The shift follows an internal review involving the White House, the Pentagon and the state department over the last month. Although the Bush administration is in deep trouble over Iraq, it remains focused on Iran. A well-placed source in Washington said: "Bush is not going to leave office with Iran still in limbo."

The White House claims that Iran, whose influence in the Middle East has increased significantly over the last six years, is intent on building a nuclear weapon and is arming insurgents in Iraq and Afghanistan.

The vice-president, Dick Cheney, has long favoured upping the threat of military action against Iran. He is being resisted by the secretary of state, Condoleezza Rice, and the defence secretary, Robert Gates.

Last year Mr Bush came down in favour of Ms Rice, who along with Britain, France and Germany has been putting a diplomatic squeeze on Iran. But at a meeting of the White House, Pentagon and state department last month, Mr Cheney expressed frustration at the lack of progress and Mr Bush sided with him. "The balance has tilted. There is cause for concern," the source said this week.

Nick Burns, the undersecretary of state responsible for Iran and a career diplomat who is one of the main advocates of negotiation, told the meeting it was likely that diplomatic manoeuvring would still be continuing in January 2009. That assessment went down badly with Mr Cheney and Mr Bush.

"Cheney has limited capital left, but if he wanted to use all his capital on this one issue, he could still have an impact," said Patrick Cronin, the director of studies at the International Institute for Strategic Studies.

The Washington source said Mr Bush and Mr Cheney did not trust any potential successors in the White House, Republican or Democratic, to deal with Iran decisively. They are also reluctant for Israel to carry out any strikes because the US would get the blame in the region anyway.

"The red line is not in Iran. The red line is in Israel. If Israel is adamant it will attack, the US will have to take decisive action," Mr Cronin said. "The choices are: tell Israel no, let Israel do the job, or do the job yourself."

Almost half of the US's 277 warships are stationed close to Iran, including two aircraft carrier groups. The aircraft carrier USS Enterprise left Virginia last week for the Gulf. A Pentagon spokesman said it was to replace the USS Nimitz and there would be no overlap that would mean three carriers in Gulf at the same time.

No decision on military action is expected until next year. In the meantime, the state department will continue to pursue the diplomatic route.

Sporadic talks are under way between the EU foreign policy chief, Javier Solana, and Iran's top nuclear negotiator, Ali Larijani, on the possibility of a freeze in Iran's uranium enrichment programme. Tehran has so far refused to contemplate a freeze, but has provisionally agreed to another round of talks at the end of the month.

The head of the International Atomic Energy Agency, Mohamed ElBaradei, has said that there are signs of Iran slowing down work on the enrichment plant it is building in Natanz. Negotiations took place in Tehran last week between Iranian officials and the IAEA, which is seeking a full accounting of Iran's nuclear activities before Tehran disclosed its enrichment programme in 2003. The agency's deputy director general, Olli Heinonen, said two days of talks had produced "good results" and would continue.

At the UN, the US, Britain and France are trying to secure agreement from other security council members for a new round of sanctions against Iran. The US is pushing for economic sanctions that would include a freeze on the international dealings of another Iranian bank and a mega-engineering firm owned by the Iranian Revolutionary Guard. Russia and China are resisting tougher measures.