On Saturday, the military said the soldier was killed in action on Friday in Iraq's vast Anbar province, one of the country's most troubled areas.
Four marines have died and more than 100 suspected Iraqi fighters have been arrested since operations began on Sunday.
US marines hit the Iraqi town of Haditha as they continued to search for fighters in the province.
Sweeping through the town in tanks and armoured cars at 2.30am (2330 GMT), they said they blew up a cache of weapons and explosives and briefly exchanged small arms fire with fighters. But resistance was lighter than expected.
Haditha, 240km west of Baghdad, had been regarded as a prime holdout for fighters because of its location on the Euphrates River. Yet the raid failed to net a big payoff in suspects.
"The fact is that there was nothing here," Lieutenant Colonel Greg Stevens said.
The marines are pushing hard to stabilise the Euphrates corridor and bring Anbar to heel in a campaign, named Operation River Blitz, launched last weekend.
Hit and run
Fighters who fled Falluja before it was taken back by US forces in November are thought to have infiltrated the area, waging a war of roadside bombs and mortar attacks and then melting back into the predominantly Sunni local population.
US marines admit the situation in
Anbar has deteriorated too far
Just how much at odds Anbar is with the US-backed government in Baghdad was shown by the tiny turnout in the country's historic election on 30 January when only 2% voted, either through fear of reprisals or antipathy to the new Iraqi order.
This may have been one of the triggers for River Blitz, and the marines admit the situation in Anbar has deteriorated too far.
On Wednesday they fought their way into the neighbouring town of Haqlaniya, a few kilometres to the south, and there had been some talk that fighters might have left there for Haditha.
But Stevens was not discouraged that the fighters had failed to appear and take on his tanks.
"The fact that we are just sitting here is a good thing. It means that they don't have the free rein of the place."
Marines were accompanied on their mission by a platoon of Iraqi soldiers, operating as a private security firm, paid and trained by the marines and called the Freedom Guard.
Washington has been desperate to improve the performance of the post-Saddam Hussein Iraqi armed forces.
An attack late on Friday in northern Iraq destroyed the pipeline which connects oil fields in Dibis with Kirkuk, about 35km to the southeast, an official of the state-run North Oil Company said on condition of anonymity.
It will take at least four days to
repair the Dibis-Kirkuk pipeline
The official said it would take at least four days to repair the line.
Pipelines around Kirkuk have been blown up repeatedly in the last few weeks.