Commanders in Iraq have decided to begin the drawdown of U.S. forces by not replacing an Army brigade when it leaves volatile Diyala province in December, The Associated Press has learned.

Instead of replacing the 3rd Brigade of the 1st Cavalry Division, soldiers from another brigade in Salahuddin province next door will expand into Diyala, thereby broadening its area of responsibility, several officials said Tuesday.

In this way, the number of Army ground combat brigades in Iraq will fall from 20 to 19, marking the start of what President Bush has billed as a shift in the American military mission away from fighting the insurgency toward more support functions like training and advising Iraqi security forces.

The December move, which has not yet been announced by the Pentagon, was described to the AP by Col. Stephen Twitty, commander of the 4th Brigade, 1st Cavalry, in a telephone interview Tuesday. It was confirmed by three other officials in Iraq, including Lt. Col. Michael Donnelly, chief spokesman for the commanding general of U.S. forces in northern Iraq, Maj. 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 U.S. 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.