British forces have been sent from Basra to the volatile border with Iran amid warnings from the senior US commander in Iraq that Tehran is fomenting a "proxy war".

In signs of a fast-developing confrontation, the Iranians have threatened military action in response to attacks launched from Iraqi territory while the Pentagon has announced the building of a US base and fortified checkpoints at the frontier.

The UK operation, in which up to 350 troops are involved, has come at the request of the Americans, who say that elements close to the Iranian regime have stepped up supplies of weapons to Shia militias in recent weeks in preparation for attacks inside Iraq.

The deployment came within a week of British forces leaving Basra Palace, their last remaining base inside Basra city, and withdrawing to the airport for a widely expected final departure from Iraq. Brigadier James Bashall, commander of 1 Mechanised Brigade, based at Basra said: "We have been asked to help at the Iranian border to stop the flow of weapons and I am willing to do so. We know the points of entry and I am sure we can do what needs to be done. The US forces are, as we know, engaged in the 'surge' and the border is of particular concern to them."

The mission will include the King's Royal Hussars battle group, 250 of whom were told at the weekend that they would be returning to the UK as part of a drawdown of forces in Iraq.

The operation is regarded as a high-risk strategy which could lead to clashes with Iranian-backed Shia militias or even Iranian forces and also leaves open the possibility of Iranian retaliation in the form of attacks against British forces at the Basra air base or inciting violence to draw them back into Basra city. Relations between the two countries are already fraught after the Iranian Revolutionary Guards seized a British naval party in the Gulf earlier this year.

The move came as General David Petraeus, the US commander in Iraq, and Ryan Crocker, the US ambassador to Iraq, made some of the strongest accusations yet by US officials about Iranian activity. General Petraeus spoke on Monday of a "proxy war" in Iraq, while Mr Crocker accused the Iranian government of "providing lethal capabilities to the enemies of the Iraqi state".

In an interview after his appearance before a congressional panel on Monday, General Petraeus strongly implied that it would soon be necessary to obtain authorisation to take action against Iran within its own borders, rather than just inside Iraq. "There is a pretty hard look ongoing at that particular situation" he said.

The Royal Welsh battle group, with Challenger tanks and Warrior armoured vehicles, is conducting out regular exercises at the Basra air base in preparation for any re-entry into the city. No formal handover of Basra to the Iraqi government has yet taken place and the UK remains responsible for maintaining security in the region.

The Iraqi commander in charge of the southern part of the country, General Mohan al-Furayji, said he would not hesitate to call for British help if there was an emergency.

While previous US military action has been primarily directed against Sunni insurgents, it is Shia fighters, which the US accuses Iran of backing, who now account for 80 per cent of US casualties.

For the British military the move to the border is a change of policy. They had stopped patrols along the long border at Maysan despite US concerns at the time that the area would become a conduit for weapons into Iraq.

The decision to return to the frontier has been heavily influenced by the highly charged and very public dispute with the United States. British commanders feel that they cannot turn down the fresh American request for help after refusing to delay the withdrawal from Basra Palace. They also maintain that the operation will stop Iranian arms entering Basra.

Brigadier Bashall said: "We are not sitting here idly at the air bridge. The security of Basra is still our responsibility and we shall act where necessary. We are also prepared to restore order in Basra City if asked to do so."

The US decision to build fortifications at the Iranian border, after four years of presence in Iraq, shows, say American commanders, that the "Iranian threat" is now one of their main concerns.

Maj-Gen Rick Lynch, commander of the US Army's 3rd Infantry Division, said 48 Iranian-supplied roadside bombs had been used against his forces killing nine soldiers. "We've got a major problem with Iranian munitions streaming into Iraq. This Iranian interference is troubling and we have to stop it," he told The Wall Street Journal this week.

Meanwhile at a conference in Baghdad on regional co-operation, Iran claimed the US was supporting groups mounting attacks from Iraqi territory in the Kurdish north.

Said Jalili , Iran's deputy foreign minister, last night said: "I think [the US and its allies] are going to prevaricate with the truth because they know they have been defeated in Iraq and they have not been successful. And so they are going to put the blame on us, on the other side."