Story Highlights• More attacks October-December 2006 than any three-month period since 2003
• Gates has signed orders to speed up deployment of additional troops
• Senate rejects timetable for troop withdrawal from Iraq
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WASHINGTON (CNN) -- A new Pentagon report said some elements of the war in Iraq fit the definition of civil war, but the term "does not adequately capture the complexity of the conflict."
The war in Iraq has been characterized by fighting between the majority Shiite Muslim sect and the minority Sunni Muslims, who were in power under Saddam Hussein's regime.
But the report also cites Shiite-on-Shiite violence; al Qaeda and Sunni insurgent attacks on coalition forces; and "widespread criminally motivated violence" as features that complicate the designation of civil war.
The quarterly report, mandated by Congress, said attacks and casualties documented for the last three months of 2006 are the highest since the war began four years ago.
The document, dated March 2, was released on Wednesday.
The congressional report cites declassified parts of the recently released National Intelligence Estimate in discussing the question.
It defines "the hardening of ethno-sectarian identities and mobilization, the changing character of the violence, and population displacements," as characteristic of civil war.
The report says warfare in Iraq has morphed from a "predominantly Sunni-led insurgency against foreign occupation to a struggle for the division of political and economic influence among sectarian groups and organized criminal activity." (Read about earlier Pentagon report)
It said the numbers of "attacks on and casualties suffered by coalition forces, the ISF [Iraqi security forces], and Iraqi civilians for the October-December reporting period were the highest for any three-month period since 2003."
The report said the attacks were concentrated in Baghdad and in Anbar, Salaheddin, and Diyala provinces, with a record 45 attacks a day in Baghdad. Compared with Baghdad, levels of attacks elsewhere were low.
"Coalition forces continued to attract the majority of attacks, while the ISF and Iraqi civilians continued to suffer the majority of casualties. Casualties from these attacks decreased slightly in January, but remained troublingly high."
The report said data collected includes "violence reported to or observed by coalition forces," so the report gives only a "partial picture of the violence experienced by Iraqis."
The report cites U.N. civilian casualty estimates reported by hospitals. "For the month of December, the U.N. estimated that more than 6,000 civilians were killed or wounded. This is about twice as many casualties as were recorded by coalition forces."
More aviation support
Defense Secretary Robert Gates has signed orders to speed up the deployment of 3,000 additional troops to Iraq, CNN has learned.
The troops primarily will be responsible for providing airborne support for the 20 brigades of ground combat forces.
The troop increase brings the number of additional forces being deployed to Iraq to more than 31,000, nearly 10,000 more than the number President Bush initially said would be needed for the "surge."
The troops will be deployed in May, 45 days earlier than planned.
In the first concrete sign that the higher level of ground forces may last into next year, the Pentagon is considering extending the tour of duty for several more units, according to military officials.
CNN's Mohammed Tawfeeq and Basim Mahdi contributed to this report.
An Iraqi soldier guards a checkpoint in Baghdad on Thursday.