LONDON - A former head of Britain's domestic spy agency has accused the US of concealing its abuse of terror suspects, stepping up an MI5 fightback over accusations that it colluded in torture.

Eliza Manningham-Buller said Tuesday she had not understood why alleged 9/11 mastermind Khalid Sheikh Mohammed had been willing to talk to American interrogators.

She said she only discovered he had been waterboarded when she read about it after her retirement in 2007.

"The Americans were very keen that people like us did not discover what they were doing," she said in a specially arranged lecture at Britain's upper house of parliament in London.

The US had been "very keen to conceal from us what was happening."

Her comments come as the spy agency hits back at claims it colluded with US counterparts in the torture of terror suspects.

The allegations were sparked by a British court's decision last month to release details of US torture of a former Guantanamo Bay inmate.

Ex-prisoner Binyam Mohamed -- who was born in Ethiopia but is a British resident -- has charged he was asked questions by US interrogators that could only have come from the British intelligence services.

Ministers and current MI5 head Jonathan Evans have strongly denied the accusations of collusion, but there were still question marks over when Britain knew about the US apparently changing its rules on torture following the 9/11 attacks.

Manningham-Buller's remarks could put the intelligence-sharing relationship between Washington and London, already strained by last month's court decision which angered the US, under further pressure.

In her comments Tuesday, she did not mention the recent case, but focused on the treatment by the US of the alleged mastermind of the September 11, 2001 attacks on New York and Washington that killed nearly 3,000 people.

She said she had wondered how, in 2002 and 2003, the US had been able to supply Britain with intelligence from Mohammed.

"I said to my staff, 'Why is he talking?' because our experience of Irish prisoners, Irish terrorists, was that they never said anything," she said.

"They said, 'Well, the Americans say he is very proud of his achievements' when questioned about it.

"It wasn't actually until after I retired that I read that, in fact, he had been waterboarded 160 times."

She said Britain had lodged "protests" with the Americans about its treatment of detainees, but gave no further details.

Common Dreams/AFP