Afghan Taliban is being secretly aided by Pakistani security services, according to a leaked NATO report.

Pakistan on Wednesday, February 1 hit out angrily at a secret NATO report accusing its spies of assisting the Taliban, saying that pre-dawn air strikes killed at least 20 local Taliban fighters.

Pakistan's alliance with the United States and NATO plummeted to an all-time low after US air strikes killed 24 Pakistani soldiers on November 26 and Islamabad has since shut its Afghan border to NATO supply convoys.

Relations with Afghanistan are also notoriously frosty over mutual blame for insurgencies plaguing both countries, but top-level talks in Kabul on Wednesday had been aimed at charting new cooperation.

But the leaked NATO document claims that Islamabad, via Pakistan's ISI intelligence agency, is "intimately involved" with the insurgency and that the Taliban assume victory is inevitable once Western troops leave in 2014.

The BBC said the report was based on material from 27,000 interrogations of more than 4,000 captured Taliban and Al-Qaeda operatives.

"Pakistan's manipulation of the Taliban senior leadership continues unabatedly," the report was quoted as saying.

Taliban captives said Islamabad was using a web of intermediaries and spies to provide strategic advice to the Taliban on fighting US and NATO troops, AFP reported.