A US intelligence assessment on Iraq says "civil war" accurately describes certain aspects of the conflict, including intense sectarian violence.
President Bush is sending 21,500 extra US troops to Iraq
However the report, compiled by US security agencies, adds that the controversial term does not adequately sum up the complexity of the situation.
The National Intelligence Estimate gives a bleak assessment of Iraq's future unless the violence is stemmed.
The White House described the report as "tough but fair".
The document uses "civil war" to describe elements of the conflict, including "the hardening of ethno-sectarian identities, a sea change in the character of the violence and population displacements".
It says violence between Sunnis and Shias is being driven by increasing polarisation within Iraqi society, compounded by a weak government and security force.
The UN says the war has produced the biggest movement of refugees in the Middle East since the creation of Israel in 1948.
However, the report points out that the conflict also includes Shia-on-Shia violence, al-Qaeda and Sunni insurgent attacks on coalition forces, and widespread violence by criminals.
US Defence Secretary Robert Gates said the term "civil war" over-simplified the situation.
"I believe that there are essentially four wars going on in Iraq: one is Shia-on-Shia, principally in the south; the second is sectarian conflict, principally in Baghdad; third is the insurgency; and fourth is al-Qaeda," he told reporters before the intelligence estimate was issued.
The report also warned of ominous consequences if the level of violence went unchecked.
"Unless efforts to reverse these conditions show measurable progress during the term of this estimate in the coming 12 to 18 months, we assess that the overall security situation will continue to deteriorate."
It adds: "Coalition capabilities including force levels, resources and operations remain an essential stabilising element in Iraq."
Bombings and mortar attacks are a daily threat to Iraqi civilians
US national security adviser Steve Hadley said the report was "a tough look at Iraq", but did not contradict the president's plans to send 21,500 additional troops.
Instead, it "explains why the president concluded that a new approach, a new strategy was required", he said.
The BBC's Ian Watson in Washington says the Bush administration is likely to use the report to justify the president's new strategy, as it concludes the Iraqi security forces will be hard-pressed to operate on their own against Shia militias.
The document argued against a quick withdrawal of US troops, saying it would fracture the Iraqi army, strengthen al-Qaeda elements in the country and significantly increase violence.
However, the Democratic leader of the House of Representatives, Nancy Pelosi, said "no number of US troops is going to change" the deadly circle in Iraq.
"Our emphasis should be on working with Iraq's leaders and Iraq's neighbours to produce the kind of political and diplomatic breakthroughs that offer the best chance to stop the violence," she said.
The report - the first National Intelligence Estimate on Iraq in more than two years - also says Iran and to a lesser extent Syria are contributing to a worsening of the situation.