In March, 35 American soldiers and marines died in Iraq, nearly all of them killed by insurgents.

In April, 52 Americans were killed.

The total in May was 80.

In those three months, insurgents killed more US troops than Saddam Hussein's army did during the fight to oust his regime.

As of Friday, 1,779 uniformed Americans had been killed in Iraq since the war began in March 2003, and all but 139 of the deaths - 1,640 of them - have occurred during the insurgency.

The insurgency's deadliness has been demonstrated even more dramatically by its targeting of Iraqi civilians and police officers. According to the Iraq Body Count, a volunteer organization of researchers and academics, at least 16,816 Iraqis were killed in the insurgency by the end of May, going back to May 1, 2003, when President George W Bush declared an end to major combat operations.

The past four months have been among the deadliest for Iraqis. Last month, 653 Iraqis were killed, an average of more than 21 a day.

''Nothing could show more clearly than the number of people being killed just how the war is going,'' said Peter Zimmerman, an American-born professor in the War Studies department at King's College London.

''Those numbers say what nobody wants to - that we in fact do not have the insurgents on their last legs.''

US Defense Secretary Donald Rumsfeld told Congress Thursday that the United States would prevail despite the increasing number of deaths. ''Any who say that we've lost this war, or that we're losing this war, are wrong - we are not,'' Rumsfeld said. ''We have recognized this is a tough business. It is difficult, it is dangerous and it is not predictable.''

But Rumsfeld said Sunday it could take as long as 12 years to defeat the insurgency in Iraq and that it will be up to Iraqi forces to do the job.

``We're not going to win against the insurgency. The Iraqi people are going to win against the insurgency,'' Rumsfeld said. ``That insurgency could go on for any number of years. Insurgencies tend to go on five, six, eight, 10, 12 years.''

Measured against past wars, the number of US service people killed in Iraq is relatively low. The war in Korea claimed more than 54,000 US troops; 58,000 died in Vietnam.

But the evident dangers and the lack of a clear end to the conflict have damaged the US military's ability to recruit new volunteers, according to Pentagon figures, and, as several polls indicate, reinforced negative attitudes in Europe and the Middle East about the United States.

The most recent monthly totals for the American and Iraqi deaths are part of an overall upward trend in casualties, indicating that the US military has encountered stronger resistance than first acknowledged by the Bush administration, which has been predicting the imminent defeat of the insurgency for more than two years.

The military's top commander in Iraq, General John Abizaid, speaking to Congress last week, reported that the insurgency is as strong as it was six months ago.

The insurgency since April has become increasingly deadly, after a two-year pattern of several active months followed by a brief period of relative quiet.

The Bush administration has resisted calls for setting a timetable to begin bringing troops home and has emphasized efforts to train an Iraqi army.

As more Iraqi battalions become stronger, the strategy calls for a gradual withdrawal of American forces.

``The report from the field is that while it's tough, more and more Iraqis are becoming battle-hardened and trained to defend themselves,'' Bush said last week.

``And that's exactly the strategy that's going to work.''

The insurgents, however, have killed increasing numbers of civilians, even as larger numbers of Iraqi troops supplement the 135,000-strong US force in the country.

``There's no way of knowing who all of them [insurgents] are, but it seems as though the biggest group is disaffected Sunnis,'' said Joost Hilterman, director of the Middle East Project of the International Crisis Group.

Others include foreign fighters and former regime members, he said.

``They won't give up,'' he said. ``The foreign fighters, especially, have been drawn to Iraq because it's an opportunity to go after Americans face-to-face, and they're willing to die to take their shots.''

The insurgents have succeeded partly by identifying and exploiting US military weaknesses - and then adapting their tactics to US defenses.

Last month, for example, there were about 700 attacks reported against US forces using improvised explosive devices, or IEDs, the highest number since the invasion of Iraq.

Insurgents have used the explosives since early in the fight, taking advantage of a US failure to provide troops with enough armored vehicles.

When more armored vehicles arrived, the insurgents were able to quickly and significantly alter bomb designs to create blasts capable of penetrating the armor.

And when the military began using jamming equipment to prevent bombs from being detonated by radio waves, the insurgents moved to infrared lasers.

``This is a serious counter-insurgency campaign that the administration was late to take seriously, and we're paying the price,'' said Anthony Cordesman, a fellow at the Center for Strategic and International Studies and a former national security adviser to Senator John McCain. ``The best we can say is that we're not losing. But the truth is we can't say we're winning.''

Despite a steady series of attacks against US troops in the early days after Saddam's fall, the Bush administration failed to react with a major show of power, according to Cordesman.

``The single biggest problem was they made no real preparations for stability operations, which allowed for chaos in Iraq, and that allowed the insurgency to build in numbers and effectiveness,'' he said. ``That doesn't mean the administration can be blamed for all that's gone wrong, because nothing like taking over Iraq had happened for us before.

``You would have expected, though, at minimum, a quicker reaction once it was obvious the war didn't end with Saddam's fall.''

On Friday, while saying that the insurgency would not end immediately, Bush said creation of Iraq's new constitution, which is to be written by August 15, will help bring stability to Iraq.

Cordesman, the former McCain adviser, said he returned from Iraq two weeks ago.

``I talked to a lot of US military, and there was no perception from anybody - from anybody - that the insurgency was being defeated,`` he said. ``They weren't at all optimistic that this thing is coming to an end any time soon.``