By Nick Childs BBC Pentagon correspondent

The US is losing soldiers at an average of 2.25 a day

The Democratic presidential contender, John Kerry, called it "a tragic milestone", and again attacked the Bush administration's policy on Iraq.

But what is the significance of this landmark?

It may have more of a psychological and political effect at home than it has a strategic impact on the ground, at least for now. But that is not to diminish it.

It could still affect the presidential election campaign, reinvigorate Mr Kerry's attacks on President Bush's policies, and re-focus concerns on the conflict - essentially bringing an end to the respite for Mr Bush of not-so-bad news from Iraq in recent months.

US military frustration

If there are worries in the Pentagon about whether the rising death toll will damage US public support for the war, they are well hidden.

For a long time, since it became clear there was no early end in sight to the insurgency, the mood in the Pentagon has been one of grim resolution.

Even behind the scenes, most officials argue that the mounting casualties only increase their determination to get the job done.

But that is not the end of it.

Behind the 1,000 deaths figure is the fact that the trend in US casualties has not really diminished, and the rate of injuries may actually have accelerated - pointing to an increasingly sophisticated insurgency and the lack of progress in defeating it.

And this landmark could feed into what feels like a growing US military frustration over how the insurgency is being handled now.

'1,000 and counting'

There is an acknowledgement that the number of safe havens for insurgents in Iraq - particularly places like Falluja, Ramadi, and Samarra in what is known as the Sunni triangle - poses a serious threat.

Bush said the Iraq mission must be completed to honour those killed

Top US military commanders say they have the military capability to deal with them if necessary.

But because of Iraqi political sensitivities, and the need to put together enough Iraqi security forces to do the job, tackling these essentially no-go areas will have to wait.

Pentagon officials talk about creating a strategy. But delaying implementing that strategy poses military and political risks as well.

Of course, while all losses are tragic, the figure of a 1,000 military deaths pales in comparison with historic conflicts like Vietnam, where 58,000 US service personnel lost their lives, or Korea, where the figure was 37,000.

But the point is that it is 1,000 and counting, and that it was not meant to be like this.

The Iraq conflict was meant to be a showcase of the US military's capabilities in the 21st Century to wage war in a different way, and overwhelm an enemy with technological superiority.

But officials now acknowledge that they miscalculated the resilience of the insurgency.