“This is a major reason for the increased levels of murders and executions.”

The findings were issued on the day Robert M Gates was sworn in as defence secretary, replacing Donald H Rumsfeld.

Gates said he planned to travel to Iraq shortly to consult with military commanders as part of a broad administration review of Iraq strategy.

But the report also held out hope that decisive leadership by the Iraqi government might halt the slide toward civil war.

While the efforts by Iraqi Prime Minister Nouri alMaliki to encourage political reconciliation among ethnic groups had shown little progress, it said that Iraqi institutions were holding and that members of the current government “have not openly abandoned the political process.”

The Pentagon assessment, titled “Measuring Stability and Security in Iraq,” is mandated by Congress and issued quarterly. The new report, completed last month, noted two parallel trends.

On the one hand, the Iraqi security forces are larger than ever, with 322,600 Iraqi soldiers, police officers and other troops, an increase of 45,000 since August.