Defense Secretary Robert Gates said Thursday the pace of a US troop withdrawal from Afghanistan starting next year remained an open question, even as he voiced optimism about the course of the war.

Speaking at a press conference on the release of a White House policy review on the war, Gates said progress was "palpable" on the ground but that it was too soon to say how many troops could be pulled out after a target date of July 2011.

"In terms of when the troops come out, the president has made clear it'll be conditions-based," Gates said.

"In terms of what that line looks like beyond July 2011, I think the answer is, we don't know at this point.

"But the hope is that as we progress, that those drawdowns will be able to accelerate."

Nearly 100,000 US troops are stationed in Afghanistan, after President Barack Obama ordered in 30,000 reinforcements a year ago in a bid to turn the tide in the nine-year-old war.

Gates said the pace of a gradual withdrawal of US and allied troops would depend in part on efforts to expand and train Afghan security forces, citing major advances in recent months.

The White House review said the troop surge had made progress in curbing the Taliban and damaging Al-Qaeda, but warned security gains won in a bloody year were "fragile" and reversible.

The review predicted a "responsible reduction" of US forces could begin next July, without indicating how many troops might depart.

The administration has said an initial drawdown might involve no more than 2,000 troops.

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