London (AFP) - The CIA tortured Al-Qaeda suspects "to the point of death" by drowning them in water-filled baths, Britain's Daily Telegraph reported on Monday, ahead of the publication of a US Senate report on interrogation techniques.

The paper quoted one security source as saying the torture of at least two suspects, including the alleged mastermind of the September 11 attacks Khalid Sheikh Mohammed, went far beyond the waterboarding admitted by the Central Intelligence Agency.

"They weren't just pouring water over their heads or over a cloth," the paper quoted the source as saying, adding: "They were holding them under water until the point of death, with a doctor present to make sure they did not go too far."

A second source cited by the paper also spoke of the treatment meted out to Mohammed, who is in US military custody in Guantanamo Bay, as well as alleged USS Cole bomber Abd al Rahim al-Nashiri, who is also being held at the detention camp on Cuba.

"They got medieval on his ass, and far more so than people realise," the source, said to be familiar with the still-classified accounts of the torture, was quoted as saying.

An upcoming report by the US Senate based on a review of classified CIA documents would "deeply shock" the public because of its graphic portrayal of the extreme interrogation techniques used by the CIA, a third source said.