Washington: The Central Intelligence Agency took top al-Qaeda suspects close "to the point of death" by drowning them in water-filled baths during interrogation sessions in the years that followed the September 11 attacks, far beyond the waterboarding details so far revealed, according to an intelligence source.

The description of the torture meted out to at least two leading al-Qaeda suspects, including the alleged 9/11 mastermind Khalid Sheikh Mohammed, far exceeds the conventional understanding of waterboarding, or "simulated drowning" so far admitted by the CIA.

US Senate intelligence committee chair Dianne Feinstein says the CIA torture program was "brutality that stands in stark contrast to our values". Credit:AP

"They weren't just pouring water over their heads or over a cloth," said the source who has first-hand knowledge of the period. "They were holding them under water until the point of death, with a doctor present to make sure they did not go too far. This was real torture."

The account of extreme CIA interrogation comes as the US Senate prepares to publish a declassified version of its so-called "torture report", a 3600-page document based on a review of several million classified CIA documents.