The US has stopped the momentum of the Taliban in Afghanistan and will begin withdrawing troops from the country next summer, an Obama administration review said today.

Taliban momentum has been "arrested in much of the country and reversed in some key areas, although these gains remain fragile and reversible", the five-page summary said.

The review indicated that the administration was "setting conditions" to begin the "responsible reduction" of troops in July.

The overview of the long-awaited report contained little evidence to back up its conclusions and is considerably more upbeat than a separate, gloomier report carried out by the US National Intelligence Estimates. This report, which represents the collective view of 16 US agencies including the CIA, said the chance of success was limited unless Pakistan tackled safe havens for the Taliban along its side of the border.

The Pentagon said the intelligence report was based on out-of-date information, and failed to take account of successes over the past few months following the troop reinforcements. The actual White House assessment document is classified and will not be made public, but interested members of Congress will be briefed on it in January.

Obama last year ordered an increase of 30,000 US troops in Afghanistan, taking the total to 100,000. Although a decision on the precise number to be withdrawn in July will not be taken until closer to the time, 20,000 could leave without making a significant difference to the conflict, yet be enough to send a signal to the American public that there is an exit strategy, according to officials.

British ministers and military commanders broadly echoed Washington's position. They say that while the timing and extent of cuts in the number of troops in Afghanistan should be conditions-based, forces could start leaving next summer and that British troops will no longer have a ground combat role after 2014.

The review says progress is most evident in the way Afghan and coalition forces are "clearing the Taliban heartland" in Kandahar and Helmand provinces, and in the increased size and capability of Afghanistan's security forces.

Afghan army and police are scheduled to grow to more than 300,000 troops over the next two years. They face an estimated 25,000 to 30,000 Taliban guerrillas and other rebels.

The document said that "the surge in coalition military and civilian resources, along with an expanded special operations forces targeting campaign and expanded local security measures at the village level, has reduced overall Taliban influence and arrested the momentum they had achieved in recent years in key parts of the country".

On al-Qaida, the review spoke of major progress in dismantling the Pakistan-based leadership of the terror network.

"Most important, al-Qaida's senior leadership in Pakistan is weaker and under more sustained pressure than at any other point since it fled Afghanistan in 2001," the report found. It warned that the US was still the principal target for al-Qaida, and that "Pakistan and Afghanistan continue to be the operational base for the group that attacked us on 9/11".

Obama was scheduled to announce the results of the review, compiled from reports submitted by military, diplomatic and intelligence officials since mid-October, in an appearance before reporters later today.

The London-based thinktank Chatham House today warned that corruption and a lack of justice was feeding support for the Taliban.

"Political marginalisation has pushed many into supporting the Taliban, while corruption has reached a point where parts of the state have been co-opted by criminal interests," said the report, No Shortcut to Stability.