The Pentagon noted that the US death rate is lower than in Vietnam

It did not identify the 2,500th casualty, in line with US policy not to release details until 24 hours after the family has been informed.

A Pentagon statement said 1,972 of those who died were killed in action.

The campaign group Iraq Body Count estimates that the number of civilians killed since the outset of the conflict ranges between 38,355 and 42,747.

It makes its calculation on the basis of media reports, and believes it to be a conservative estimate.

Other reports put the number of civilian casualties much higher.

Thousands of Iraqi security forces, military personnel from other countries, and Iraqi and foreign insurgents have also died.

'Tremendous sacrifice'

Between November 2005 and March 2006, the number of US military fatalities fell month-by-month, as insurgents seemed to focus attacks on Iraqi security forces and civilians.

However, they jumped back above the average for April and May. The Pentagon put the blame on a recent surge in rebel violence.

Polls in the US suggest opposition to America's presence in Iraq has grown - while support for President George Bush's handling of the war has fallen.

The Pentagon notes that the death rate for US troops is much lower than those in the Vietnam or Korean wars, to which Iraq has sometimes been compared.

In each of those conflicts more than 50,000 US troops died.

"It's important to remember that there is a mission, and there is a greater good which sometimes necessitates tremendous sacrifice," said the US army's Brig Gen Carter Ham.

"Rather than focus on an aggregate number, I think it's more important for us to remember that there are individuals in that aggregate number... to whom we should be very, very grateful, and to their families."

The US Senate on Thursday approved $65.8bn for military operations in Iraq and Afghanistan, following a similar vote in the House of Representatives on Tuesday.

Since the 9/11 attacks, the US has now spent or allocated $438bn on its "war on terror", with more than 70% spent in Iraq.