Iraqi troops inflicted heavy casualties on U.S. Marines in an ambush in the southeastern city of Nasiriya, peppering U.S. forces with bullets and artillery. Thousands of civilians trying to flee Basra Friday were fired upon by Iraqi paramilitaries. Three U.S. Marine infantry battalions occupy the northern and southern parts of Nasiriya. Scores of coalition soldiers were dropped deep into Iraq Friday in what is being called the longest helicopter-borne air assault operation ever.