The death of six US troops this week has made 2007 the most deadly year for US forces in Iraq.
US casualties peaked in the first half of 2007 but declined recently
Five US soldiers and one sailor were killed in three separate incidents, bringing the number of deaths in 2007 to over 850.
With almost two months to go, US losses have already surpassed those of 2004 - previously the worst year.
The milestone comes despite a recent sharp drop in US and Iraqi casualties due to the surge in US forces.
Five US soldiers were killed on Monday in two separate roadside bomb attacks, in Kirkuk and Anbar provinces.
A US sailor also died on Monday from injuries sustained in an explosion in Salahuddin province, north of Baghdad.
Military officials attribute the high number of deaths to an initial increase in combat operations, and higher visibility of US troops on the streets earlier this year as part of President George W Bush's "surge" strategy.
The strategy, which saw an extra 30,000 US troops sent to Iraq, included sending troops out of large bases and into more dangerous communities.
Casualties peaked as the US and Iraqi forces launched numerous operations to bring Baghdad and outlying areas under control.
The US military has reported a decline in casualties since the build-up of troops was completed in June, bringing the total number of US troops in Iraq to over 160,000.
Lt-Gen Raymond Odierno, the second-ranking US commander in Iraq, told a Pentagon briefing last week there had been a five-month decline in combat deaths.
Insurgent attacks, including roadside bomb blasts, had been on a downward trend since June, Lt-Gen Odierno said.
Previously the worst year for US combat deaths was 2004, which saw heavy fighting with Sunni insurgents in Falluja.
To date 3,857 US troops have been killed since the invasion of Iraq in 2003, according to the independent icasualties.org website, which monitors US troop deaths.