The U.S. military said in a secret report that the Taliban, backed by Pakistan, are set to retake control of Afghanistan after NATO-led forces withdraw, raising the prospect of a major failure of Western policy after a costly war.

Lieutenant Colonel Jimmie Cummings, a spokesman for the NATO-led International Security Assistance Force, confirmed the existence of the document, reported on Wednesday by Britain's Times newspaper and the BBC.

But he said it was not a strategic study.

The classified document in question is a compilation of Taliban detainee opinions, he said. It's not an analysis, nor is it meant to be considered an analysis.

Nevertheless, it could be interpreted as a damning assessment of the war, dragging into its 11th year and aimed at blocking a Taliban return to power.

It could also be seen as an admission of defeat and could reinforce the view of Taliban hardliners that they should not negotiate with the United States and President Hamid Karzai's unpopular government while in a position of strength.

The U.S. military report could boost the Taliban's confidence and make its leaders less willing to make concessions on demands for a ceasefire, and for the insurgency to renounce violence and break ties to al Qaeda.

But Britain's Kabul Ambassador William Patey wrote on his Twitter feed that if elements of the Taliban think that in 2015 they can take control of Afghanistan they will be in for a shock. He did not say if he was referring to the document.

Hours after the Times report, the Afghan Taliban said that no peace negotiation process had been agreed with the international community, particularly the Americans.

Taliban spokesman Zabihullah Mujahid said in a statement that prior to any negotiations, confidence building measures must be completed, putting pressure on Washington to meet demands for the release of five Taliban in U.S. custody.

The hardline Islamist movement also said it had no plans to hold preliminary peace talks with Afghanistan's government in Saudi Arabia, dismissing media reports of talks in the kingdom.

The U.S. military said in the document that Pakistan's Inter-Services Intelligence (ISI) security agency was assisting the Taliban in directing attacks against foreign forces.

Reasserting control over the country would be more difficult a second time for the Taliban, however, with Afghan police and soldiers expected to number about 350,000 beyond 2014 and some foreign troops likely to remain, including elite forces.

Close U.S. ally Australia said on Wednesday that its special forces could be in the country for years beyond the handover, with other allies likely to take a similar stance.

The report overshadowed a visit to Kabul by Pakistani Foreign Minister Hina Rabbani Khar designed to repair ties and raise the issue of peace talks with the Taliban with Karzai.

I can disregard this as a potentially strategic leak ... This is old wine in an even older bottle, she told reporters, reiterating Pakistan's denials it backs militant groups.

Khar, whose visit was the first high-level meeting in months between officials from both countries, added that the neighbours should stop blaming each other for strained cross-border ties.

The Times said the highly classified report was put together by the U.S. military at Bagram air base, near Kabul, for top NATO officers last month. It was based on interrogations of more than 4,000 Taliban and al Qaeda detainees, it said.

Captain John Kirby, a Pentagon spokesman, declined to comment on the specifics of the report, but did acknowledge long-standing concerns about the ties between elements of the ISI and the Taliban. This is not a new notion.

Still, he told reporters, we really do believe that militarily we are making an impact on the Taliban.

Large swathes of Afghanistan have been handed back to Afghan security forces, with the last foreign combat troops due to leave by the end of 2014. But many Afghans doubt their security forces will take firm control once the foreign troops leave.

NATO spokeswoman Oana Lungescu, speaking in Brussels, played down the implications and said a surge offensive had seen the Taliban suffer tremendous setbacks.

We know that they have lost a lot of ground and a lot of leaders, and we also know that support for the Taliban is at an all time low, she said.

As of January 1, 889 U.S. soldiers had been killed in a conflict that was launched after the September 11, 2001 attacks and has drained almost half a trillion dollars from U.S. coffers.

WRONG POLICIES

New accusations of Pakistani collusion with the Taliban will likely further strain ties between Western powers and Islamabad.

Critics say Pakistan uses militants as proxies to counter the growing influence of India in Afghanistan. The belief that Pakistan supports the insurgents is widely held in Afghanistan.

It would be a mistake now for the international community to leave Afghanistan, and drop us in a dark ocean, said Afghan telecommunications worker Farid Ahmad Totakhil.

Pakistan is reviewing ties with the United States which have suffered a series of setbacks since a U.S. raid that killed Osama bin Laden on Pakistani soil in May last year humiliated Pakistan's powerful generals.

A November 26 cross-border NATO air attack that killed 24 Pakistani soldiers deepened the crisis, prompting Pakistan to close supply routes to NATO forces in Afghanistan.

Pakistan is seen as critical to U.S. efforts to stabilise Afghanistan, a feat one foreign power after another has failed to accomplish over the country's turbulent history.

Islamabad has resisted U.S. pressure to go after insurgent groups like the Taliban, and argues Washington's approach overlooks complex realities on the ground.

Pakistan says the United States should attempt to bring all militant groups into a peace process and fears a 2014 combat troop exit could be hasty, plunging the region into the kind of chaos seen after the Soviet exit in 1989.

They don't need any backing, Tariq Azim, of the Pakistani Senate's Defence Committee, told Reuters, referring to the Taliban. Everybody knows that after 10 years, they (NATO) have not been able to control a single province in Afghanistan because of the wrong policies they have been following.

The Taliban announced this month they would open a political office in Qatar to support possible reconciliation talks. There has been talk of efforts to hold separate talks in Saudi Arabia.

U.S. lawmakers, in a hearing on Wednesday, also pressed the Obama administration to step up measures to ensure Western soldiers are not attacked by Afghan forces or employees of security firms working with NATO.

France said it would withdraw its troops completely by the end of 2013 after four of its soldiers were killed by a rogue Afghan soldier, the latest such insider attack.

The Defense Department said that over 40 similar attacks on foreign personnel had taken place since mid-2007, some of them by people working with private security contractors.

We ... owe it to our military personnel to do everything we can to reduce this sort of risk, said Rep. Adam Smith, the senior Democrat on the House Armed Services Committee.

Pentagon officials said NATO takes extensive steps to vet Afghans working with foreign troops and is exploring ways to prevent future attacks.

(Additional reporting by Dan Magnowski, Rob Taylor and Amie Ferris-Rotman in KABUL, David Brunnstrom in BRUSSELS, Qasim Nauman in ISLAMABAD, Missy Ryan in WASHINGTON; Writing by Michael Georgy and Rob Taylor; Editing by Robert Birsel and Cynthia Osterman)