The marines were on a foot patrol outside of Fallujah, just west of Baghdad, when they were attacked on Thursday, said a US marines statement.

"The patrol was attacked with an IED (improvised explosive device) fashioned from several large artillery shells," said the statement.

It said seven of those injured in the incident had returned to duty.

The dead were were from the 2nd Marine Division's Regimental Combat Team 8, the statement said, adding that the names were withheld pending notification of next of kin.

Another US marine was killed in Fallujah on Wednesday.

The attack was the deadliest on US forces in Iraq since an August 3 blast killed 14 marines during combat operations near Haditha, 260 kilometers (160 miles) northwest of Baghdad.

More than 2,110 US service personnel have died in Iraq since the March 2003 invasion.

The latest attack was announced as about 500 US and Iraqi troops launched a new offensive against insurgents in Ramadi, also west of Baghdad.

The offensive came a day after residents of Ramadi, capital of Al-Anbar province, reported a sudden insurgent presence on the streets, with dozens of fighters roaming unchecked and putting up Al-Qaeda posters.

Fallujah was known as an Al-Qaeda stronghold when in November 2004, it was stormed by thousands of US and Iraqi soldiers and pounded by US artillery, planes and tanks.

Another report says, despite US claims of progress in quelling the insurgency in Iraq, it remains as robust as ever with a potential for becoming a good deal stronger, according to a new study released Thursday.

The study by two veteran defense analysts working for the Washington Institute for Near East Policy also said the US operation in Iraq was at a "tipping point" that will last for six to nine months.

"I think the outcome of this tipping period is probably going to dictate whether or not the US effort in Iraq succeeds or fails," analyst Jeffrey White said at a lunch unveiling the report.

The study said the insurgency, comprising of nationalists, members of Saddam Hussein's toppled regime and foreign Islamic fighters, showed no sign of losing steam 32 months after the US-led invasion.

"Although thousands of insurgents have been killed and tens of thousands of Iraqis have been detained ... incident and casualty data reinforce the impression that the insurgency is as robust and lethal as ever," it said.