The last six months of 2004 proved the deadliest period for US forces in Iraq despite the formal end of the US-led occupation in June, with a total of 503 soldiers killed, figures showed Friday.
Rather than deflating the insurgency, the first half year of Iraqi sovereignty under the US-backed interim government saw a surge in bloodshed.
The deadliest month was November when 141 troops were killed, reflecting the heavy combat in the Sunni Muslim bastion of Fallujah where US troops battled rebels in the street in some of the heaviest fighting ever in Iraq.
In December, a total of 75 soldiers were killed, 14 of them in last week's unprecedented suicide bombing of a US army base in the northern city of Mosul - the deadliest single strike ever on Americans in Iraq.
Another 67 lost their lives in October, 87 in September, 75 in August and 58 in July. The tolls include combat and non-combat deaths.
The formal US occupation in Iraq was dissolved on June 28, with power handed to an interim Iraqi government, selected under the auspices of the United States and the United Nations.
US officials hoped the formal transfer of sovereignty in Iraq would take wind out of the insurgency's sails. Instead, violence has escalated against both US troops and Iraqis.
In the first half of the year, with Iraq under occupation, 401 soldiers died in Iraq.
Observers says the violence testifies to the insurgency's staying power and evolution.
"There is no question the growing death toll indicates a growing insurgency," Jorst Hiltermann, Middle East director of the International Crisis Group, said.
US commanders say the high casualty rate reflects the insurgency's increasing use of car bombs and booby trap explosives.
"When you have people using car bombs to target convoys and locations, they have the ability to choose their time and place," said Lieutenant Colonel Steve Boylan, who had said in September that 60 percent of all insurgent attacks were either bombings or mortar or rocket fire.
But he also attributed the mounting death toll to fact US forces had taken a more aggressive stance in recent months.
In the later part of the year, US troops fought to reclaim cities like Fallujah, Najaf and Samarra that had slipped into the control of armed groups.
"Whenever there is a signicant increase in combat operations, you expect casualties to rise," he said.
Still, Boylan acknowledged that the fight in those places and the continuing battle to control those cities was brutal in nature.
"It (fatalities) is the unfortunate byproduct of what we're doing. Fighting an insurgency is the toughest and hardest type of operation. Cities make it even tougher," Boylan said.
However, in volatile northern Babel and restive al-Anbar provinces, home to Fallujah and Ramadi, US marines were reporting a dramatic drop in attacks.
"Since the Fallujah battle kicked off, we've seen a 60 percent decrease in the overall number of attacks in our entire area of operations," said US Marine spokesman Lieutenant Lyle Gilbert.
US Brigadier General Erv Lessel reported a similar drop across Iraq, with attacks against US forces down by 50 percent from November. He said attacks had slipped from highs earlier this year around 100 attacks per day, down to around 60 per day.
But while the insurgents were attacking the US military less, Iraqis working with the Americans were still coming under fire.
Since Tuesday, more than 100 Iraqi security forces, public servants and civilians have been killed, including the deputy governor of al-Anbar province.
Boylan acknowledged the trend.
"We've seen a decrease in attacks against our forces and an increase in attacks against police and civilians."