Overall, the year of 2007 was the bloodiest year of the Iraq war, according to the latest figures released by U.S. and Iraq authorities.

During the year, at least 899 U.S. troops died in Iraq, the highest annual toll since the American-led invasion in March 2003, according to Pentagon figures released by the independent website icasualties.org Tuesday.

The Iraqi Ministry of Health said 16,232 civilians died last year, comparing to the 2006 death toll of 12,320.

However, last December has probably been the safest month for U.S. forces in Iraq since the 2003 invasion and the least deadly for Iraqi civilians in the last 12 months.

On the military front, 21 U.S. personnel died in Iraq during December, making the average daily death tally last month the lowest since the start of the war.

The Iraqi Ministry of Health said 481 civilians died nationwide last month in war-related violence such as bombings, mortar attacks and sectarian slayings, making it the least deadly month for Iraqi civilians of 2007.

After considerably higher monthly death tolls earlier in 2007, the number of fatalities among Iraqi civilians and U.S. troops has been decreasing since the American military completed a troop buildup in June.

But few were celebrating the recent downturn in violence as proof of irreversible progress.

U.S. military and political officials are warning that recent months' security gains have opened the door to new challenges, some of which could spawn fresh violence as the Iraqis jostle to reclaim their lives.