WASHINGTON (Reuters) - Defense Secretary Robert Gates could approve plans for a major build-up of U.S. forces in Afghanistan before President-elect Barack Obama takes office on January 20, officials say.

A U.S. helicopter lands at a military base in Ghazni province October 30, 2008. REUTERS/Ahmad Masood

Faced with an intensifying Taliban insurgency, U.S. military planners are working to identify a prospective force of more than 20,000 combat and support troops requested by commanders in Afghanistan, mainly for duty in the poppy-growing South, where the need for more Western forces is greatest.

Some estimates have called for a U.S. force as large as 30,000 troops, officials say, similar in size to the 2007 surge of U.S. forces in Iraq that is credited with helping drive violence levels there down sharply.

Meanwhile, Obama’s repeated call for more U.S. forces for the war has led the Pentagon to expect little resistance from the incoming administration, officials said.

“The president-elect has already indicated that he wants to put additional troops in Afghanistan,” said a senior defense official, who spoke on condition of anonymity because the plans have not yet been finalized.

By making the decision soon, Gates could begin preparing to deploy forces without actually having to move troops before January 20, the official said. That would let the new president move quickly to send additional troops once he takes office, or halt the deployments if he prefers, the official said.

Defense officials say a main obstacle to approval is the ability of planners to locate the desired forces at a time when the military structure remains under strain with 146,000 U.S. troops still in Iraq.

“Final approval comes when we tell the secretary: ‘Yeah, we can provide them,’” said another official.

As in Iraq, the force’s main task would be to enhance security to make way for reconstruction and development projects including the building of local governing institutions capable of resisting the Taliban.

WILL SURGE WORK?

But current and former U.S. officials have warned that a surge of forces into Afghanistan will not necessarily meet with the same success as the troop build-up in Iraq.

“Additional troops in Afghanistan may be necessary but they will not, by themselves, be sufficient to lead to the results we saw in Iraq. A similar confluence of events that contributed to success in Iraq does not appear to exist in Afghanistan,” former Defense Secretary Donald Rumsfeld said in a recent newspaper opinion piece.

The addition of 20,000 to 30,000 U.S. forces would expand the current 70,000-strong Western military presence in Afghanistan to more than 90,000 troops.

The United States now has 32,000 troops in Afghanistan, with 14,000 under NATO command and 18,000 under U.S. command.

But some analysts believe the United States ultimately will need more than 100,000 troops to stabilize Afghanistan before the Afghan army is ready to take over security.

“I suspect that to succeed in Afghanistan, we’re eventually going to have to swing a sizable fraction of what we now have in Iraq into Afghanistan,” said Stephen Biddle of the Council on Foreign Relations.

“The scale of the shift will be large, and the time needed to pull it off will be long,” he said.

The Bush administration has announced plans to send an Army combat brigade to Afghanistan in January. Gates said last week he wants to send another three brigades of combat ground forces and an aviation brigade, beginning as early as next spring.

A main Pentagon objective is to insert enough U.S. forces into Afghanistan to safeguard elections late in the year.

Analysts said the plan provides enough U.S., NATO and Afghan forces to secure polling sites and establish checkpoints against suicide bombers in the volatile East and South.

Seth Jones of RAND Corp., said Afghan and U.S. officials would also need the security cooperation of local tribes and Afghanistan’s neighbors if the election is to succeed.

“This has to be done in cooperation with Pakistan and Iran in trying to ensure that those countries are supporting the elections, because both have provided support to the Taliban and a few other insurgent groups,” Jones said.

Afghanistan’s lack of infrastructure means the U.S. military must build bases, landing strips, supply depots and other infrastructure necessary to support a larger force.