A stark assessment released on Thursday by the nation’s intelligence agencies depicts a paralysed Iraqi government unable to take advantage of the security gains achieved by the thousands of extra American troops dispatched to the country this year.

The assessment, known as a National Intelligence Estimate, casts strong doubts on the viability of the Bush administration strategy in Iraq. It gives a dim prognosis on the likelihood that Iraqi politicians can heal deep sectarian rifts before next spring, when American military commanders have said that a crunch on available troops will require reducing the United States’ presence in Iraq.

But the report also implicitly criticises proposals offered by Democrats, including several presidential candidates, who have called for a withdrawal of American combat troops from Iraq by next year and for a major shift in the American approach, from manpower-intensive counterinsurgency operations to lower-profile efforts aimed at supporting Iraqi troops and carrying out quick-strike counterterrorism raids.

Such a shift, the report says, would “erode security gains achieved thus far” and could return Iraq to a downward spiral of sectarian violence. After a summer of rancorous debate over the future of America’s mission in Iraq, the intelligence report is the most prominent and authoritative assessment to date of what the administration calls a surge strategy.

The report, which represents the view of America’s 16 intelligence agencies, suggests that policy makers face a dilemma. While the current strategy in Iraq has produced “measurable but uneven improvements” in security, it says, the approach has done little to bridge sectarian divides in Iraq. The report also says that pulling American troops out of Iraq would most likely make things far worse.