The report emerged at the end of a week in which several hundred people were killed in violence, including 270 killed near the town of Najaf when US and Iraqi forces battled what Iraqi officials say were members of a shadowy sect.

The National Intelligence Estimate report, parts of which were obtained by Reuters, described a serious situation, with Iraqi-on-Iraqi violence having surpassed al-Qaeda activities.

It said elements of the conflict could be called "civil war."

"The term 'civil war' does not adequately capture the complexity of the conflict in Iraq," said the report. "Nonetheless, the term 'civil war' accurately describes key elements of the Iraqi conflict."

US President George W. Bush's administration has steadfastly avoided using the term "civil war," which could heighten already growing calls for an early withdrawal of US troops from Iraq.

Portions of the 90-page report seen by Reuters predicted the situation in Iraq would worsen unless efforts were made to reverse conditions.

Bush said in January he would send 21,500 additional US troops to Iraq in an effort to get a grip on the sectarian killings and insurgent attacks, especially in Baghdad.

The increase in troops -- on top of some 130,000 US troops already in Iraq -- is widely seen as a final attempt to avert all-out sectarian civil war between Iraq's Shia Muslim majority and Sunnis once dominant under Saddam Hussein.

The report said a rapid withdrawal of US forces would lead to massive civilian casualties and the possible intervention by Iraq's neighbours, including a military incursion by Turkey.

It said the Iraqi government would be unlikely to survive as a non-sectarian institution and that al-Qaeda would use parts of Iraq to plan attacks inside and outside the country.