RAMSTEIN AIR BASE, Germany - The Pentagon plans to extend its buildup of combat troops in Afghanistan, initially announced as lasting until late spring, well into next year, a senior U.S. military official said Friday.

The move comes as U.S. and allied commanders anticipate a renewed offensive this spring by the Taliban.

The decision means American troop levels will remain roughly the same into 2008 rather than returning to the 22,000-to-23,000 level that had prevailed through much of last year. Troop numbers increased this month to about 26,000, the highest of the war, with the arrival of a brigade of the 82nd Airborne Division,

The decision, expected to be announced in Washington as early as next week, entails sending an army combat brigade to replace the 3rd Brigade of the 10th Mountain Division when it leaves this spring. If no replacement were sent, the only full combat brigade in Afghanistan would be the 82nd Airborne brigade.

The 3rd Brigade originally was to go home this month, as it turned over command to the brigade of the 82nd Airborne, but the Pentagon announced in January that it instead will be kept in place four extra months.

Until now it was not clear whether the Pentagon would replace the 3rd Brigade when it went home.

The senior official, who spoke on condition of anonymity because the decision to extend the buildup has not been announced, said the unit that replaces the 3rd Brigade in late spring will remain for a year. Thus the number of brigades in Afghanistan, which increased from one to two this month with the 82nd Airborne's arrival, will remain at that level until the spring of 2008 and possibly beyond, he said.

The official declined to identify the replacement brigade. There are about 3,500 soldiers in a brigade.

The increase in the U.S. force comes as NATO's new top commander, Gen. John Craddock, is seeking 1,500 to 2,000 extra combat troops for the Afghanistan mission, plus about 800 more from the British.

Canada has about 2,500 troops in the country.

Decisions about adjusting U.S. troop levels next year will depend in part on whether the United States' NATO partners send all the combat and support forces they have promised.