Security has been tightened at US embassies ahead of the release of a report, which is expected to reveal details of harsh interrogation techniques used by the CIA.

It is likely to include graphic details about how the intelligence agency used methods, such as waterboarding, on terrorism suspects in the years after the September 11th attacks.

The 480 page document was compiled by the Senate intelligence committee.

Reuters says the report describes how senior al-Qaeda operative Abdel Rahman al-Nashiri was threatened by his interrogators with a buzzing power drill.

It adds that the drill was never actually used on Nashiri, who was suspected of masterminding the bombing of the USS Cole in 2000.

In another instance, the report documents how at least one detainee was sexually threatened with a broomstick.

White House spokesman Josh Earnest said: "There are some indications that...the release of the report could lead to greater risk that is posed to US facilities and individuals all around the world."