Experts predict a shift in language could deepen domestic discontent with the US involvement in Iraq.
Asked at a news conference in Estonia on Tuesday what the difference was between the current bloodshed and civil war, George Bush, the US president, said the latest bombings were part of a nine-month-old pattern of attacks by al-Qaeda fighters aimed at fomenting sectarian violence by provoking retaliation.
Stephen Hadley, the White House national security adviser, said the Iraqis "don't talk of it as a civil war" because the army and police had not fractured along sectarian lines and the government continued to hold together.
US officials' reluctance to use the words "civil war" is more than a semantic difference.
The phrase carries a political dimension because it could further weaken Americans' support for a war that has already helped remove Bush's Republican party from control of the US congress.
Violence between Sunnis and Shia has increased dramatically this year.
Multiple bombings in a Shia neighbourhood of Baghdad last Thursday killed more than 200 people and drew reprisal attacks in Sunni districts.
Experts predict a shift in language could deepen domestic discontent with the US involvement in Iraq
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Analysts say the US public will not tolerate troops being used as referees between warring Iraqi factions.
MSNBC, NBC's cable network, on Tuesday displayed a graphic reading "Iraq: The Civil War" in its coverage.
Other US networks said they would continue reporting under broader terms, such as "war in Iraq".
Chris Simpson, American University communications professor, said the shift in coverage reflects a growing consensus among foreign-policy experts that the conflict is a civil war.
"When those elites shift, the media typically follows," Simpson said. "To some extent, the media do play a role in shaping that opinion, but mostly they follow it."
The Los Angeles Times said it had adopted the term in October "without public fanfare," making it the first major news outlet to do so.
The New York Times said it would use the term sparingly and not to the exclusion of other labels, as the conflict also has elements of an insurgency, an occupation, a battle against terrorism and "a scene of criminal gangsterism".
The Washington Post said it has no policy to describe the conflict. CNN, ABC and CBS said some of their correspondents have referred to the rising sectarian violence as a civil war, or examined the debate among experts over whether the term is appropriate.