The following correction was printed in the Guardian's Corrections and clarifications column, Thursday December 21 2006





The Bush administration is weighing options for a naval build-up in the Gulf as a show of force and a warning to Iran on its nuclear programme and its support for Shia militias in Iraq, it emerged yesterday.

Under the proposed build-up, first reported by CBS television, the Pentagon would send an aircraft carrier to join one already in the region. The proposed deployment was described as a message to Tehran not to take provocative steps, and was not preparation for an attack.

The idea of sending a second aircraft carrier was raised this month by the commander of US forces in Iraq, General John Abizaid. But it also comes amid mounting pressure from Saudi Arabia against a withdrawal of US forces from Iraq.

Pentagon officials were considering Gen Abizaid's request, but few other details were immediately available. A Pentagon spokesman said there would be no comment on military movements.

"The administration has been pretty clear about Iran's role in the region, which is that Iran has to stop being provocative," the White House spokesman, Tony Snow, told reporters.

The aircraft carrier USS Dwight D Eisenhower has been in the region since September, along with four other ships and submarines carrying 6,500 sailors. The navy could move other carriers into the region within six weeks. The USS Stennis, a cruiser which was scheduled to deploy in early 2007 in any event, would be the most likely ship to be deployed.

However, if the US were to contemplate a military strike, it would need far more than two carriers, said Reva Bhalla, an analyst at Strategic Forecasting Inc. The US deployed five carriers ahead of the invasion of Iraq in 2003.

Outgoing UN secretary general Kofi Annan said yesterday that military intervention in Iran would be "unwise and disastrous", as the Security Council debated a resolution that would impose sanctions on Tehran over its nuclear programme. "I believe that the council, which is discussing the issue, will proceed cautiously and try and do whatever it can to get a negotiated settlement," he said.

The latest draft resolution would order all countries to ban the supply of specified materials and technology that could contribute to Iran's nuclear programmes.

Reports that the US is leaning towards an even stronger posture against Iran reflects indications that Washington wants to deepen its military presence in the region. It follows warnings from Saudi Arabia that it would fund Sunni militias in Iraq in the event of a US troop withdrawal. "The aircraft carrier is a way of assuring the Saudis that the inclination is to do more rather than less, and that we are not going to leave them in the lurch," said John Pike, a military analyst.

The tougher posture on Iran and a temporary troop surge in Iraq would both run counter to the findings of the Iraq Study Group earlier this month which recommended a withdrawal of US combat forces from Iraq by early 2008, and the opening of diplomatic talks with Iran and Syria.

George Bush has yet to unveil his new strategy for Iraq, postponing a planned policy address until the new year. However, a series of leaks suggest that the White House is in favour of sending an extra 20,000 to 30,000 more troops to Iraq for the next six or eight months. Bush told Washington post on Tuesday he plans to expand the size of the US military to deal with the long-term fight against terrorism.

The Pentagon believes the US can only bolster its forces in Iraq by extending deployments, a course that would eventually wear down the army.

Meanwhile, the administration is concerned at evidence that Shia militias are receiving support and training from Iran.

A Pentagon report on Monday said that the Mahdi Army, the armed militia loyal to the radical Shia cleric Moqtada al-Sadr, had eclipsed al-Qaida as the most dangerous agent of sectarian violence.

The quarterly assessment from the Pentagon was the bleakest to date, describing the security situation in Iraq as "tragic". Violence rose by 22% over the past three months to a new high of 959 attacks a week against US and Iraqi forces, and the Pentagon warned that the continued bloodshed was eroding the legitimacy of the prime minister, Nouri al-Maliki.

The growing power of Shia militias has come to occupy a greater share of US attention, and fuelled frustration with Mr Maliki. The Iraqi prime minister is dependent on both militia, and so far has resisted US pressure to confront them.

Escalating violence

The Pentagon report said attacks on US and Iraqi troops and Iraqi civilians jumped in recent months to the highest level since Iraq regained its sovereignty in June 2004.

· From mid-August to mid-November, the weekly average number of attacks increased 22% from the previous three months

· There were an average of 959 attacks a week on US and Iraqi forces during the three months ending in November

· Ninety-three Iraqi civilians are killed or injured every day. Attacks against Iraqi security forces reached a new high at 33 a day

· Injuries and deaths among US forces stood at 25 a day.