Nine US marines have been killed and nine injured in what is thought to be a bomb attack near the Iraqi city of Falluja, the US military has said.
Operations near the town of Falluja have increased over the past day
In a statement, the officials said the deaths occurred in the western Anbar province on Saturday.
The rebel-held city of Falluja is being threatened with a major military strike by US and Iraqi forces.
The interim Iraqi government is continuing peace talks with Falluja representatives this weekend.
The soldiers, from the 1st Marine Expeditionary Force, were killed while carrying out what US military officials called "increased security operations" in the province, which also includes the insurgent stronghold of Ramadi.
Eight were initially reported to have been killed, but the army issued a new death toll of nine on Sunday.
The casualty figures are the highest for the US in a single attack in more than six months.
Details of the incident remain sketchy.
US forces have been carrying out a heavy bombardment of Falluja in preparation for a full-scale attack.
They say the Sunni-dominated city has become a safe haven for rebels linked to Jordanian rebel Abu Musab al-Zarqawi.
A US official quoted by the Associated Press news agency confirmed that air strikes were continuing, targeting buildings believed to be used by Zarqawi's followers.
News agencies report that US officials estimate there are up to 2,000 fighters entrenched in the city.
Peace talks to avert an assault on Falluja, believed to have started on Wednesday, are being held by a government-backed delegation and leaders from the rebel-held Sunni city.
Interim Prime Minister Iyad Allawi said the talks represented the final chance for Falluja to avoid an assault.
'No good news'
He has demanded that foreign militants be expelled from Falluja and Iraqi forces, backed by American troops, be allowed into its centre.
But our correspondent in Baghdad, Claire Marshall, says the talks are apparently making little progress and a spokesman for a Sunni clerical association in Falluja said there was no good news.
In mid-October, Mr Allawi demanded that the city hand over Zarqawi, who heads the newly renamed al-Qaeda Organisation for Holy War In Iraq, or face invasion.
Many of the most high-profile attacks across Iraq, both suicide bombings or kidnappings, have been claimed by Zarqawi's group.
Falluja leaders have repeatedly said that they have no idea of his whereabouts.
US Commander Denis Hejlik says his forces are poised to attack.
"We are gearing up to do an operation," he told reporters.
"When we are told to go, we will go. When we do go, we'll whack them."