* UK sticks to 2011 timeline to begin pulling out troops
* Britain shrugs off WikiLeaks criticism of its troops
* Gates arrives just before U.S. strategy review begins
By Mohammed Abbas and David Alexander
KABUL, Dec 7 (Reuters) - British Prime Minister David Cameron, on an unannounced visit to Afghanistan, said on Tuesday he was confident British troops could start leaving early next year, when transition to Afghan forces beings.
Britain has the second-biggest foreign troop contingent in Afghanistan after the United States, and Cameron, who was on his sixth visit to Afghanistan, has said he wants British troops out of combat roles by 2015.
While he has said before he wants that process to begin next year, British commanders have since tried to play down the prospect of any major withdrawals in early 2011, saying it would depend on the readiness of Afghan forces to take over and other conditions on the ground.
"What I've seen on this visit gives me confidence that our plans for transition are achievable beginning early next year," Cameron told a news conference in Kabul alongside Afghan President Hamid Karzai.
As Cameron and Karzai spoke, U.S. Defense Secretary Robert Gates arrived at the sprawling Bagram Air Base just north of the capital, Kabul.
Gates's latest visit comes just as U.S. President Barack Obama reviews his Afghanistan war strategy, and a few days after the president made a trip himself to Afghanistan.
Gates will also meet Karzai and U.S. and NATO commanders.
"(The trip) is taking place just as the National Security Council is in the midst of its evaluation of that strategy, so clearly what the secretary learns here, what he sees here, what he takes from here, will inform the discussion that is taking place back in Washington," Pentagon press secretary Geoff Morrell told reporters travelling with Gates.
At a conference in Lisbon last month, NATO leaders agreed to meet Karzai's timeline for foreign troops to end combat operations in Afghanistan by the end of 2014. Some U.S. and NATO leaders have warned that may spill into 2015.
The NATO summit agreement has thrown the spotlight onto the readiness of Afghanistan's roughly 260,000 police and soldiers to take over from foreign forces.
Some foreign commanders acknowledge there are problems with the training, equipment and retention rates among Afghan forces and that a target of 306,000 for Afghan forces by October 2011 will be hard to meet.
Cameron and Chief of the Defence Staff General David Richards, the head of Britain's armed forces, both painted an upbeat picture of progress in training the Afghan army.
"I think though next year, as we've agreed, it's conditions-based but looking at the progress we've made, I was only here three months ago it's quite astronomical how quickly things are coming together," Richards said.
SUCCESS AT A PRICE
U.S. commanders have also reported signs of progress in the war since September, when the United States completed deploying the 30,000 additional troops Obama authorised last year. The United States now has about 100,000 troops in Afghanistan out of a total foreign contingent of about 150,000.
Military and civilian casualties however are at their worst levels since the Taliban were ousted in late 2001, despite the presence of a record number of foreign troops.
Commanders have talked up recent success and say they have broken the Taliban's momentum, but the increasingly unpopular war is widely seen as going badly for Washington and its allies and European NATO leaders, like Cameron, are under pressure to withdraw troops.
Both Cameron and Richards said improving conditions could allow British troops to start withdrawing next year. Cameron has passionately defended his deadline of having no British troops in combat operations by 2015.
Britain has about 9,500 troops in Afghanistan, the bulk of which are in Helmand, a Taliban stronghold in the south, where they were spread thinly until Obama authorised extra U.S. troops, 20,000 of which are in southern provinces.
British troops have since been able to concentrate on smaller, strategic areas of Helmand.
At least 346 British troops have been killed in Afghanistan since 2001, almost a third of them this year. Casualty rates among foreign troops have risen dramatically, particularly in the south and east, since July 2009 as NATO-led forces mounted more operations against Taliban-led insurgents.
Cameron, who arrived on Monday, shrugged off recent criticism of the troops' performance, saying they had been spread too thinly before the extra U.S. troops arrived.
The apparent criticism, which said British troops "were not up to the task" of securing Helmand, came from Afghan and US. embassy comments obtained by the WikiLeaks website and published over the past week by several media outlets.
"When you look at what was said, it was relating to a previous period, when we all know now there weren't enough troops in Helmand," Cameron said in Helmand late on Monday.
(Writing by Paul Tait; Editing by Miral Fahmy) (If you have a query or comment on this story, send an email to news.feedback.asia@thomsonreuters.com)
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