President Barack Obama has told US troops in Afghanistan they are making crucial "progress" against insurgents.

During an unannounced visit to Bagram air base near Kabul, Mr Obama said: "Today we can be proud that there are fewer areas under Taliban control."

He also spoke to President Hamid Karzai for about 15 minutes by telephone.

The visit comes a year after Mr Obama ordered a troop "surge" in Afghanistan, but weeks after a Pentagon report found violence there at an all-time high.

The US president's national security team is to report on the new strategy later this month.

Mr Obama's trip to Afghanistan comes about one year after he announced at West Point Military Academy the US would strengthen combat troops in Afghanistan with 30,000 reinforcements.

'New phase'

Mr Obama flew to Bagram air base to thank US soldiers for their service during the American holiday season.

He said there would be "difficult days" ahead in their fight against the Taliban.

But he added: "You're achieving your objectives, you will succeed in your mission.

Analysis Images of a leather-jacketed Commander-in-Chief telling the Screaming Eagles of the 101st Airborne Division that America is "the land of the brave because it is the home of the free" will play well at home in this holiday season - coming soon to a campaign video near you. The stars of the show for once were the meteorologists who advise the president's military advisers. They decided it was too windy for Mr Obama to make the journey by helicopter to visit President Hamid Karzai in Kabul. It would have been their first face-to-face session since it emerged from the Wikileaks cables that the Americans consider him "weak, erratic and prone to believing paranoid conspiracy theories". We'll never know how it would have gone. Still, although they weren't touched on today, the US faces tough strategic choices as it plans to start handing more security responsibility to the Afghan authorities. Expect more in the coming weeks.

"We said we were going to break the Taliban's momentum. That's what you're doing."

He also said he was looking forward to "a new phase next year, the beginning of transition to Afghan responsibility".

Mr Obama had been due to meet President Karzai face-to-face during his brief visit to Afghanistan.

But bad weather prevented him from travelling to Kabul, forcing the two men to speak via telephone, the White House said.

There has been increasing tension between the US and its Afghan allies.

Mr Karzai has complained about American military tactics, and criticised the decision to start withdrawing US troops in July next year.

And a memo by a US diplomat revealed by the Wikileaks website described Mr Karzai as having a paranoid world view.

Mr Obama's decision to send 30,000 extra troops to Afghanistan was designed to lead to a handover of security duties to Afghan police and troops.

But in a report to Congress issued last month, the Pentagon said violence had reached an all-time high, with clashes up fourfold since 2007.

The report said progress had been "uneven", with only modest gains against the insurgents.

It is Mr Obama's second visit to Afghanistan as president - the first was in March.