The number of US troops to die in Iraq since the invasion began five years ago has reached 4,000, after an attack in southern Baghdad killed four soldiers. The milestone is likely to strengthen calls for US forces to be withdrawn from the country.

The roadside bomb — the single most deadly weapon deployed by insurgents against US forces — tipped the balance of American deaths over the 4,000 mark, a week after the conflict entered its sixth year.

The US military played down the significance of the new threshold reached after a day of bombings and rocket fire across the country. At least 60 Iraqis were killed and many more were wounded.

“No casualty is more or less significant than another; each soldier, Marine, airman and sailor is equally precious and their loss equally tragic,” Rear-Admiral Gregory Smith said.

Iraq timeline: five years of conflict

From accusations that the weapons of mass destruction dossier had been sexed-up to withdrawal of British troops from Basra

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Major Brad Leighton, another military spokesman, deemed the media focus on landmark numbers inappropriate. “This isn't a lottery,” he said. But for American soldiers patrolling Iraq's streets, daily life is a lottery with death, never knowing when the next buried bomb might erupt beneath their vehicle or a mortar hit their base.

In the early days of the conflict, Iraqi guerrillas mostly used rocketpropelled grenade launchers or small arms looted from Saddam Hussein's abandoned weapons dumps to carry out random attacks. They quickly moved on to improvised explosive devices, or IEDs, often using US military manuals translated into Arabic to hone their skills.

As the battlefield evolved, Iraqi snipers also started to attack US patrols, while the IEDs developed into explosively formed penetrators, or EFPs, that fire molten copper through even the thickest armour, and which the Americans said were supplied by Iran.

In addition to the rising death toll, at least 29,000 US servicemen and women have been wounded in the conflict. Ninety-seven per cent of the casualties occurred after President Bush stood before a “Mission Accomplished” banner on the USS Abraham Lincoln in May 2003 and declared that main combat operations were over.

Even those figures are dwarfed by the number of Iraqis killed since the invasion. According to Iraqi Government estimates, 12,000 of its own security forces have been killed.

Figures for an overall Iraqi death toll vary greatly as the US military did not bother to keep records in the early days of the war. A combined World Health Organisation and Iraqi government study in January concluded that between 104,000 and 223,000 Iraqis had died violently since the invasion.

The independent Iraq Body Count website, based solely on incidents published by the media, reported close to 90,000 deaths by this month, a quarter of whom died last year when Mr Bush sent in an extra 30,000 troops to Iraq, bringing the total to 160,000.

The 1,000th US soldier died in September 2004 in the midst of a presidential election that returned Mr Bush to office for a second term. The toll climbed to 2,000 in October 2005 as Sunni Arab insurgents battled to oust the Iraqi Government, and 3,000 was reached in December 2006. Mr Bush declared on the eve of the fifth anniversary that the US was on track for victory, while acknowledging the “high cost in lives and treasure”.

The increase in troop numbers last year and the defection of tens of thousands of insurgents to local militias fighting erstwhile al-Qaeda allies has led to a dramatic drop in violence over the past nine months.

However, the level of bloodshed has crept up in recent weeks. Rockets and mortar bombs pounded the Baghdad green zone at the weekend, injuring at least five people. Up to 17 Iraqi civilians were killed by rounds that fell short of the sprawling compound that houses the Iraqi Government and the US and British embassies.

In a reminder of the other casualties of war who rarely get mentioned, the FBI said yesterday that the remains of two American contractors kidnapped more than a year ago had been found. Ronald Withrow worked for JPI Worldwide when he was abducted in January 2007. John Roy Young, who disappeared in November 2006, was employed by Crescent Security Group.

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