Commanders in Iraq have decided to begin the draw down of US forces in volatile Diyala province, marking a turning point in the US military mission.

Instead of replacing the third Brigade of the first Cavalry Division soldiers from another brigade in Salahuddin province will expand into Diyala.

In this way, the number of US Army ground combat brigades in Iraq will fall from 20 to 19.

It will also mark the start of what President George W Bush has billed as a shift in the American military mission.

The December move, not yet announced by the Pentagon, was described by Col Stephen Twitty, commander of the fourth Brigade, first Cavalry, in a telephonic interview.

Three other officials confirmed it in Iraq, including Lt Col Michael Donnelly, chief spokesperson for the commanding general of US forces in northern Iraq, Major Gen Benjamin Mixon.

The idea is to avoid vacating a contested area, like Diyala, while beginning Bush's announced reduction of at least 21,500 troops, of which 17,000 were sent to the Baghdad area last spring.

The shift in Diyala in December could be a model for follow-on reductions next year, with a redrawing of the US lines of responsibility so that a departing brigade has its battle space consumed by a remaining brigade.

At the same time, Iraqi security forces would assume greater responsibility.