BAGHDAD, Iraq (AP) - The U.S. ambassador to Iraq expressed regret Wednesday over the fatal shooting by a Marine of a wounded and apparently unarmed man in a Fallujah mosque but said the incident would not undermine U.S. efforts to remove guerrillas from the city.

Images of the shooting over the weekend, aired widely on Al-Jazeera television, have angered Iraqis - particularly the Sunni Arab minority - and Arabs across the region. American and Iraqi authorities have been trying to stem the outrage.

The U.S. military said Tuesday it was investigating the shooting after pool video footage taken Saturday by NBC showed the incident during an operation of the 3rd Battalion, 1st Marine Regiment.

``No one can be happy about an incident such as that one, but the important point is that the individual in question will be dealt with,'' Ambassador John Negroponte told reporters. ``The matter is being looked into and whatever measures are necessary under our system of military justice I know will be taken.''

``But I don't think that (the incident) in any way is a reflection on the quality and caliber of absolutely fine young service men and women we have serving here in Iraq,'' he said during his first visit to Sadr City, a large Shiite Muslim district of Baghdad.

The shooting occurred when a Marine unit entered the mosque and found five men wounded in fighting at the site the day before, when another Marine unit clashed with gunmen apparently using the mosque to fire from, according to NBC's Kevin Sites, the pool correspondent who took the footage.

Negroponte also said the fight for Fallujah - for months the main insurgent bastion - was important for the U.S. mission in Iraq, even though guerrilla leaders remain on the loose, resistance has not been crushed and attacks have increased throughout central Iraq.

``I think the liberation of Fallujah is extremely important,'' he said. ``Former regime elements and foreign fighters have been deprived of a sanctuary and a base they were using as a platform for conducting terrorist activities throughout the rest of the country, especially here in Baghdad.''

The purging of militants from the insurgent stronghold west of Baghdad also ``sets the stage for the entry of humanitarian assistance and resumption of economic activities.''

``We have over $100 million worth of reconstruction projects and that all awaits the completion of liberation of the city,'' Negroponte said. ``Hopefully, Fallujah will return to a much better life.''

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