The marines "were killed in action during a fire fight" on Wednesday in the town of New Ubaydi, near the border with Syria, a US military statement said.
"During the engagement, 16 enemy fighters were confirmed killed," it added.
Earlier the US military announced that three US soldiers "were killed when their patrol struck an improvised explosive device northwest of Baghdad" on Tuesday.
Another Marine was killed on Tuesday "by a suicide vehicle-borne improvised explosive device attack (car bomb) while conducting combat operations against the enemy", the military said.
The marine was killed near the town of Al-Karmah, just outside the restive Sunni Arab city of Falluja, west of Baghdad.
he US military announced on Thursday that another soldier had died of wounds sustained in a roadside bomb explosion in northwest Baghdad.
The soldier, who was not immediately identified, was wounded on Tuesday and died on Wednesday.
The lastest death brings to at least 2080 the number of US military personnel killed in Iraq since the March 2003 invasion, according to an AFP tally based on the independent Iraq Coalition Casualty Count.
US and Iraqi forces are pursuing an 11-day-old assault on alleged al-Qaida fighters in western Iraq.
US and Iraqi forces are carrying
out an offensive in western Iraq
Thousands of US and Iraqi troops have been attacking through towns near the Syrian border as part of Operation Steel Curtain, which was extended on Monday to the town of Ubaydi, on the Euphrates river, 20km from the Syrian border.
A US military statement said a total of about 80 fighters had been killed in Ubaydi, 30 of them since Monday night in what it said was some of the heaviest fighting of the offensive so far, with the fighters apparently feeling cornered.
"Most of those terrorists were targeted by coalition air strikes, although some were killed in direct small-arms fire engagements," it said.
Around 150 suspected fighters have been detained in the operation over the past 10 days.