Outgoing US ambassador to Iraq Zalmay Khalilzad has said he held talks last year with insurgent groups to try to bring them into the political fold.
Mr Khalilzad is set to take the post of US ambassador to the UN
He told the New York Times he had talks both before and after the bombing of a Shia shrine in February last year that sparked a surge in sectarian unrest.
The paper said Mr Khalilzad flew to Jordan for some of the talks, but they foundered after the Samarra attack.
Separately, the US military said bombs killed another five troops on Sunday.
Mr Khalilzad said: "There were discussions with the representatives of various groups in the aftermath of the elections, and during the formation of the government before the Samarra incident, and some discussions afterwards as well."
Although the paper said Mr Khalilzad declined to give more details, it identified two leading Sunni factions - the Islamic Army of Iraq and the 1920 Revolution Brigades - as having been approached.
Mr Khalilzad leaves Iraq this week to become, if confirmed, the US ambassador to the UN.
The US has begun an operation against al-Qaeda in Ramadi
Mr Khalilzad said the situation in Iraq had "not gone as well as one would have clearly liked".
He said: "I think the complicating factor was the intensification of sectarian violence, particularly in the aftermath of Samarra."
The paper quoted one US official as saying: "We were never able to find people who could reduce the violence... The insurgency itself does not have anything resembling a unified command."
Mr Khalilzad said the US and Iraq had to consider amnesties for certain groups.
"This is something that we and Iraqis, the government, will do together, and there are various types of amnesties," he said.
Separately in Iraq, the US military said four soldiers were killed on Sunday in a roadside bombing in the province of Diyala and another died in an attack in north-western Baghdad.
More than 3,200 US troops have died since the invasion in March 2003.
The violence came as US and Iraqi forces continued their security push in the Iraqi capital to try to curb insurgent and sectarian violence.
The operation by thousands of US and Iraqi forces has reduced the number of sectarian shootings but stopping the daily car bombings has proved more difficult, correspondents say.
The US military also said on Monday it had begun an operation to remove al-Qaeda militants from the city of Ramadi, west of Baghdad.
The operation came a day after Iraqi police and tribal forces clashed with al-Qaeda fighters in the city.
Ramadi has seen some of the fiercest fighting since the US-led invasion.