US Defence Secretary Robert Gates told US troops in Afghanistan their sacrifices were paying off Tuesday, ahead of a key strategic review of the nine-year war.

He flew in on the previously unannounced visit amid further tensions between Washington and Kabul after leaked American diplomatic cables offered a scathing account of Karzai and his corruption-tainted allies.

Last December US President Barack Obama ordered 30,000 extra American troops into Afghanistan and renewed efforts to build up Afghan security forces as part of a sweeping strategy shift designed to hasten an end to the fighting.

In the eastern province of Nangarhar, Gates thanked troops from the 101st Airborne and expressed sympathy over recent deaths after six Americans were killed last week by an Afghan border policeman during a training session.

It was the deadliest attack of its kind by insurgents who have infiltrated Afghan forces, which threaten to undermine a key plank of the US war strategy.

"I know you all have had a rough go of it," and taken "a lot of losses", Gates said. But he said the sacrifices were "worth it".

"What you're doing is incredibly important," he said.

His spokesman Geoff Morrell earlier said that Gates arrived "feeling very good about the progress that has been made in the past year" and said the visit would help shape a war strategy review under way in the White House.

Gates was scheduled to hold talks with Afghan President Hamid Karzai and the top NATO commander in Afghanistan, US General David Petraeus, who expressed doubts Monday about the prospect of a victory in Afghanistan by 2014.

In an interview with ABC television, Petraeus would not say he was confident that an Afghan government and its security forces would be able to take over from the US-led coalition four years from now, as envisaged by Washington.

"I don't know that you say confident. I think no commander ever is going to come out and say 'I'm confident that we can do this,'" Petraeus said.

Obama on Friday made his second trip since taking office nearly two years ago to Afghanistan — where he reassured US troops that they were winning the war but did not meet Karzai owing to what officials called bad weather.

Gates also visited troops in the eastern province of Kunar on Tuesday.

General John Campbell, commander in the eastern region, said US forces may withdraw from remote mountain outposts and redeploy to towns and villages, a recent strategy enacted in other parts of the country.

"We can't fight every single valley," said Campbell, after meeting Gates.

The Americans have already closed several outposts in the rugged border region and in the Taliban's southern heartlands, and Campbell said more would be closed in coming months as coalition forces seek to safeguard key towns.

He did not say how many bases would be abandoned out of the roughly 140 currently in operation in the east, where US-led troops are struggling to cut off insurgent supply lines from sanctuaries across the border in Pakistan.

Campbell told reporters there had been intense fighting with insurgents in the area in recent weeks.

"This is a very, very tough area… We dropped nine bombs here yesterday," said the general, referring to air raids.

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