BRUSSELS (AP) — The U.S. force in Afghanistan is undergoing a major restructuring that will bring virtually all American troops under NATO command, a top U.S. military official said Tuesday.

Vice Adm. Greg Smith, the top military spokesman in Afghanistan, said the intent was to integrate most of the 20,000 U.S. troops currently serving in eastern part of the country under separate command known as Operation Enduring Freedom, into the 100,000-strong NATO-led International Security Assistance Force.

NATO officials noted that the practical effect of this move will be to streamline and simplify the command structure, since both forces are already under the operational control of the senior commander in Afghanistan Gen. Stanley McChrystal.

The reorganization will not require the deployment of any new forces, and McChrystal will still command all allied troops in the theater, Smith said.

"It's just a matter of moving things from one account in the ledger to another," he said. "For the military we clearly need unity of command so that elements on the battlefield are not working at cross-purposes with each other."

Secretary of Defense Robert Gates signed off on the reorganization before he visited Kabul last week, Smith added.

U.S. Marine Corps personnel in Afghanistan, currently under the U.S. Central Command, will also now come under McChrystal's direct control, he said.

Operation Enduring Freedom, set up after the 9/11 attacks, covers army, navy and air force units involved in anti-terrorist operations in Afghanistan, the Philippines and the Horn of Africa.

ISAF was established in 2002, and currently numbers about 100,000 troops — nearly 60,000 of them Americans. About 40,000 new reinforcements, three-quarters of them Americans, are already being deployed to Afghanistan.

Smith said that once the reorganization is complete, only small special forces detachments and a prison guard unit will remain outside the NATO command structure.

"NATO did not want to take over issue of detention, which remains in U.S. hands," Smith told reporters at NATO headquarters in Brussels.

Smith said that the new structure was not an attempt to bring Special Forces operations, have been blamed for causing numerous civilian casualties, under firmer central control. McChrystal has made it priority to avoid causing unnecessary casualties since local resentment resulting sparked by civilian deaths has been a key factor in swelling the ranks of the Taliban insurgency.

"We're out hunting and targeting the Taliban every single day, but not (inside the villages) because that will result in civilian casualties," Smith said. "If an armed robber runs into a home, no police in the world will go in and blow up that house."