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By David Morgan

WASHINGTON, Aug 8 (Reuters) - U.S. Defense Secretary Robert Gates is expected to unify the military command for U.S. troops in Afghanistan and has endorsed a multibillion-dollar plan to nearly double the size of the Afghan army, officials said on Friday.

The changes are part of the Pentagon's latest response to a growing challenge from the Taliban insurgency, particularly in eastern Afghanistan, where officials say U.S. and NATO forces are battling militants trained at safe havens in neighboring Pakistan.

U.S. officials are looking at ways to speed more U.S troops to Afghanistan, including the possible redeployment of combat brigades now earmarked for Iraq.

But so far, the Pentagon has opted to enhance the force already on the ground by extending tours for more than 3,000 Marines while deploying about 175 support troops including helicopter units to boost the Marines' fighting capacity.

In another action intended to make U.S. forces more effective, Gates is expected soon to sign an order putting the NATO commander in Afghanistan, U.S. Army Gen. David McKiernan, in charge of nearly all U.S. forces that are not connected with NATO's 53,000-strong International Security Assistance Force.

The United States has 34,000 U.S. troops in Afghanistan, including about 15,000 already under McKiernan's command as part of ISAF. The anticipated change would give him most of the remaining 19,000 troops currently under the separate U.S. command known as Operation Enduring Freedom.

The NATO and OEF missions would remain separate, but that the switch would minimize confusion within the U.S. command ahead of expected troop deployments, possibly next year, officials said.

"The number of our troops in Afghanistan is only going up, and so we want to make sure that as we're growing, that we have the most efficient and effective command structure in place," said Geoff Morrell, Gates' press secretary.

Gates is discussing the change with his NATO counterparts and could order the command change before Aug. 31, according to an official who spoke on condition of anonymity.

FORCE EXPANSION

Meanwhile, a dramatic expansion of the Afghan National Army would help meet security needs in the first half of the next decade, officials said.

"More Afghan forces are needed to defeat the terrorists and to protect the Afghan people and to help the government expand its reach in the country," Pentagon spokesman Bryan Whitman told reporters.

The force expansion, which has already been proposed by the Afghan government, would create an active-duty Afghan National Army of 122,000 troops between 2010 to 2014, up from a current 65,000 troops. An additional pool of 12,000 Afghan trainees would boost the overall size of the army to 134,000.

U.S. and Afghan officials are discussing with NATO allies ways to cover an anticipated five-year cost of $17 billion to fund Afghan army operations, expand the force and upgrade the Afghan air force.

The Pentagon could not say how much the army expansion alone would cost. It estimated that it would cost $1 billion to add 10,000 troops and $100 million to sustain them for a year.

The expansion would also require more U.S. trainers for the Afghan military. About 15,000 U.S. troops in Afghanistan are currently devoted to training Afghan forces.

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