The Taliban, backed by Pakistan, are set to retake control over Afghanistan after NATO-led forces withdraw from the country, Britain's Times of London newspaper reported on Wednesday, citing a report compiled by U.S. forces.

It described the report as secret and highly classified, saying it was put together by the U.S. military at Bagram air base in Afghanistan for top NATO officers last month. The BBC also carried a report on the leaked document on its website.

Many Afghans are already bracing themselves for an eventual return of the Taliban, the Times said, quoting the report. Once ISAF (NATO-led forces) is no longer a factor, Taliban consider their victory inevitable, it quoted the report.

The document stated that Pakistan's security agency was assisting the Taliban in directing attacks against foreign forces -- a charge long denied by Islamabad.

Its findings were based on interrogations of more than 4,000 Taliban and al Qaeda detainees, the Times said, adding however the document was scarce on identifying individual insurgents.

A State Department spokesman and Britain's Foreign Office both declined comment on the report. NATO and Pakistani officials could not be immediately reached for comment.

Despite the presence of over 100,000 foreign troops, violence in Afghanistan is at its worst since the Taliban were ousted by U.S.-backed forces in 2001, according to the U.N.

The NATO-led International Security Assistance Force (ISAF) says levels of violence are falling.

Citing the same report, the BBC reported on its website that Pakistan and its Inter-Services Intelligence (ISI) agency knew the locations of senior Taliban leaders and supported the expulsion of foreign invaders from Afghanistan.

Senior Taliban leaders meet regularly with ISI personnel, who advise on strategy and relay any pertinent concerns of the government of Pakistan, it said.

The Times, in its report, said the document suggested the Taliban were gaining in popularity, partly because the austere Islamist movement was becoming more tolerant.

It quoted the report: It remains to be seen whether a revitalised, more progressive Taliban will endure if they continue to gain power and popularity. Regardless, at least within the Taliban, the refurbished image is already having a positive effect on morale.

(Reporting by Stephen Mangan Editing by Maria Golovnina)