Pakistan reached an understanding with the United States on drone strikes targeting Islamist militants and the attacks can be useful, according leaked remarks from a former intelligence chief.

Pakistan publicly condemns US missile attacks on Taliban and Al-Qaeda operatives as a violation of its sovereignty, but the new revelations are the latest sign of double-dealing in private.

They come in findings of a Pakistani investigation into how Al-Qaeda leader Osama bin Laden evaded detection for nearly a decade, which were published by the Al-Jazeera news network Monday.

Ahmed Shuja Pasha, who headed Pakistan's premier Inter-Services Intelligence (ISI) agency at the time of bin Laden's killing in 2011, told investigators that drone strikes had their uses.

"The DG (director general) said there were no written agreements. There was a political understanding," the report said.

The Americans had been asked to stop drone strikes because they caused civilian casualties, but "it was easier to say no to them in the beginning, but 'now it was more difficult' to do so," it quoted the former spymaster as saying.

"Admittedly the drone attacks had their utility, but they represented a breach of national sovereignty. They were legal according to American law but illegal according to international law," the report quoted the ISI chief as saying.

He also confirmed that Shamsi air base, in southwestern Pakistan, had been used for US drone strikes against people in the country.

Pakistan ordered US personnel to leave the base after botched US air strikes killed 24 Pakistani soldiers in November 2011.

His interviews also laid bare extraordinary levels of distrust between Pakistan and the United States, particularly in 2011 when relations plummeted over the US raid that killed bin Laden and a CIA contractor who shot dead two Pakistanis.

Pasha said US arrogance "knew no limits" and accused the Americans of waging "psychological warfare" over the whereabouts of Taliban leader Mullah Omar and bin Laden's successor Ayman al-Zawahiri.

He quoted a US intelligence officer as saying "you are so cheap... we can buy you with a visa," and said himself that systemic failures showed Pakistan was a "failing state".

The Pakistani report condemned the US raid as an "American act of war" and said the military should have responded much more quickly to a three-hour operation, 100 miles inside its territory.

It was Pakistan's "greatest humiliation" since East Pakistan seceded in 1971, it said.