"Taliban commanders, along with rank-and-file members, increasingly believe their control of Afghanistan is inevitable," the report first excerpted by The Times of London and the BBC said.
"Though the Taliban suffered severely in 2011, its strength, motivation, funding and tactical proficiency remains intact," the "State of the Taliban 2012" report said.
The Jan. 6 report, based on 27,000 interrogations of 4,000 Taliban and other captives, was put together by the U.S. Army at northern Afghanistan's Bagram Airfield for top NATO officers, the BBC said.
Its leaked publication coincided with Wednesday's announcement by U.S. Defense Secretary Leon Panetta that U.S. forces would step back from a combat role in Afghanistan as early as mid-2013, shifting to a training role more than a year before all U.S. troops are scheduled to withdraw.
"Many Afghans are already bracing themselves for an eventual return of the Taliban," the NATO report said.
The Afghan government "continues to declare its willingness to fight, yet many of its personnel have secretly reached out to insurgents, seeking long-term options in the event of a possible Taliban victory," it said.
The Taliban, a Sunni Islamist group, ruled Afghanistan between 1996 and 2001, with one of the strictest interpretations of Shariah law ever seen in the Muslim world.
The regime was ousted by an Oct. 7, 2001, U.S.-led invasion after the Sept. 11, 2001, terrorist attacks. Washington said the invasion would remove the Taliban regime from power and create a viable democratic Afghan state.
A spokesman for the NATO-led coalition played down Wednesday's leaked report, saying NATO did not necessarily accept the views of the Taliban detainees as valid.
State Department spokeswoman Victoria Nuland said in Washington the report would not change U.S. efforts to repair its badly strained relationship with Pakistan, particularly after a mistaken cross-border attack killed 24 Pakistani soldiers Nov. 26, 2011.
"It was not designed for any purpose other than to help those in the field understand what Taliban detainees were saying," Nuland said. "So it was in no way designed to impact on our ongoing efforts to be back on track with Pakistan."
Pakistani Foreign Minister Hina Rabbani Khar rejected the report as "old wine in an even older bottle." She told reporters her country had no hidden agenda, did not support militant groups and would back a peace process between Kabul and the Taliban, as long as it was driven by Afghans and not outsiders.
The Taliban said they would begin no peace negotiations until "confidence-building measures" were completed.
The Taliban are demanding the release of five senior members held at the U.S. military prison at Guantanamo Bay, Cuba -- including a "high-risk detainee" held for more than a decade.
The Obama administration has said it is considering the demand.
The Taliban are holding U.S. Army Pfc. Bowe Bergdahl, 25, kidnapped in Afghanistan in June 2009. He is the only U.S. soldier held by the Islamist militant and political group.