Sophisticated strikes threaten British withdrawal from Iraq

The two guards worked for the private security company Control Risks, which said they would not be named until next of kin had been informed. A local police officer said two Iraqi children among a group of onlookers which gathered at the scene were seriously injured when a second device exploded five minutes later.

British forces believe a new wave of guerrilla attacks in southern Iraq is being supplied and backed from Iran. The 1,000-strong Task Force Maysan, stationed south of Amara, has noticed a dramatic change in the "quality" of attacks on them this summer.

Last year British troops fought daily gun battles with Shia militiamen loyal to the maverick cleric, Moqtada al-Sadr. This year the number of attacks has dropped, but the attackers appear to have had some highly professional training - almost certainly from militias and elements of the Revolutionary Guard across the border in Iran.

"Last year terrorists fought pitched battles in the open, with AK-47s and rocket-propelled grenades," said Lt Colonel Andrew Williams, commanding the battle group of the Staffordshire Regiment in Maysan. "They have now changed tack, and learned from last year. They've been trained somewhere else, and their weaponry is not locally produced - some of it is state of the art." The guerrillas now carefully orchestrate their attacks after tracking every move of British patrols by mobile phones, according to soldiers.

On 10 July two soldiers and a lieutenant of the Staffords were killed when their Land Rover patrol was hit by three mortars in the west of Amara after they were lured to the area by a small decoy explosion. The mortar bombs, linked to a car battery triggered by a remote switch or radio signal, were the biggest and most sophisticated remotely triggered device seen so far.

Following the attack, Col Williams ordered a halt to patrols in Amara on foot or in lightly-protected Land Rovers until further notice. No fewer than five groups claimed responsibility for the ambush, all identified with Moqtada al-Sadr and his nominees of the Imam Hussein List, which controls the Maysan provincial council. While the cleric has promised to follow the democratic route, not all his supporters agree.

British forces were due to leave Maysan and the neighbouring desert province of al-Mathana early next year, after the general election due to be held in Iraq on 15 December. But they are concerned by recent discoveries of good-quality munitions, including RPGs, land mines, triggering mechanisms and explosives, being brought across the border from Iran.

An articulated lorry recently stopped by Iraqi border police was full of rockets, grenades and launchers. The arms, and training in their use, are believed to have been provided by local militias and units of the Revolutionary Guard across the border, though not necessarily sanctioned by Tehran.

The British forces hope to have built up the local Iraqi army and police units sufficiently for them to take over security in Maysan and al-Mathana by next year. But while army training is going ahead, the police are proving more problematic - particularly as it is recognised that more than a quarter of them are loyal to Mr al-Sadr.

International officials say current plans call for British-led international forces to start scaling back in the other two southern provinces, Basra and al-Dikaar, by the end of March. "That's when the government contracts from the Foreign Office and Department for International Development run out," said one adviser, "and that seems the time to go." Yesterday's attack may deal a further blow to such plans, however.