On The Scene: Fighting For An-Nasiriyah

(CBS)  CBS News Correspondent John Roberts is traveling with the Marines and filed this report somewhere north of an-Nasiriyah.

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As the U.S. Marines charged north toward Baghdad, they bypassed the city of an-Nasiriyah, which commands a key crossing of the Euphrates River.

And that left it to other Marines to deal with the city's determined defenders.

In the heaviest fighting of the war for Marines with the 2nd light armored reconnaissance battalion, Iraqi forces have closed in on their command post, another attempt to bloody the nose of Marines as they pushed northwards.

Iraqis harassed the reconnaissance battalion's command post all night. Marines fired back with their light armored vehicles.

In the end it was the Iraqis who paid dearly. We counted 25 dead, most of them in one skirmish, most of them from a bus Marines said was filled with Iraqi conscripts looking to reinforce the battle south in an-Nasiriyah.

The Marines may control a section of an-Nasiriyah and a critical bridge across the Euphrates River, but when we drove through early this morning they were fighting fiercely to hang onto it.

The Marines appear to have decided not to take an-Nasiriyah, just protect the supply route north but securing that route is proving a challenge. One convoy on the road was taking sniper fire while an artillery company to the north was being ambushed.

Marine commanders told CBS News weeks ago an-Nasiriyah would be the start of the tough going. But as one Marine told me, an-Nasiriyah showed them one thing: that this war isn't going to be the cakewalk everyone thought it would be.