The buildup in the increasingly violent campaign could amount to more than 20,000 troops rather than the originally planned 10,000, two senior defense officials said Wednesday on condition of anonymity because no new figures have been approved.
The newest calculations reflect growing requests from field commanders in recent weeks for aviation units, engineers and other skills to support the fighting units, the officials said.
Officials had been saying for months that they needed more people to train Afghan security forces and two more combat brigades - a total of some 10,000 people.
Commanders later increased that to the trainers and three combat brigades - or some 15,000, when extra support is included.
Now, military planners say that the number may have to grow yet again by another 5,000 to 10,000 support troops. They would be helicopter units, intelligence teams, engineers to build more bases, medical teams and others to support the fight in the undeveloped nation, where forces have to work around rugged terrain and a lack of infrastructure.
The growing numbers being quoted for the buildup in Afghanistan are not unusual.
President Bush announced in January 2007 that he would send up to 20,000 additional troops to Iraq for what since has become known as the "surge." But the number eventually grew to 30,000 by the time commanders added requests for all the military police, additional aviation needs and other support they wanted.
In Afghanistan, it is far more difficult for troops to operate in the undeveloped nation, which lacks roads, runways and facilities to support troops. And commanders in Afghanistan do not consider this a short-term surge in troops but rather the number that will be needed over a longer period, one official said.
It is unclear whether the number will win approval. Some officials believe it's unwise to build too large a force in Afghanistan, where there is long-held hostility to the presence of foreign forces.
If that large a force is approved, it's also unclear where the Pentagon would get that many extra troops for the Afghan campaign - and how quickly they could be sent.
The Defense Department already has approved the deployment of about 4,000 people - one additional Marine combat battalion and one Army brigade to be sent by January.
But with some 150,000 forces committed in Iraq, the U.S. has not had the available troops to send to Afghanistan. Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, has often noted that in Afghanistan "we do what we can, in Iraq we do what we must."
The military shortfall in Afghanistan has been a common complaint from commanders. The number of U.S. troops in Afghanistan has grown from fewer than 21,000 two years ago to more than 31,000 today.