The head of the CIA has apologised to US politicians after an internal investigation found agents had accessed Senate computers to monitor a government probe into CIA torture.

The CIA's inspector general on Thursday found officers searched the computers without permission when the Senate intelligence committee was investigating the agency.

The findings come months after the agency's director, John Brennan, told members of the Senate intelligence committee in March that the CIA had not spied on committee investigators. "Nothing could be further that the truth," he said in March.

Brennan on Thursday apologised to members of the committee.

The scandal centres on the CIA's computer archive, RDINet, which made classified documents available to committee officials investigating allegations of excesses committed by CIA officers.

In March, the committee accused the CIA of penetrating this network during a Senate investigation, an apparent breach of the US constitution's separation of legislative and executive arms of government.

The Senate committee has been investigating alleged excess CIA practices, including harsh interrogation methods such as simulated drowning, or "water-boarding", to question captured suspects following the September 11, 2001 attacks.

The White House is expected to deliver a declassified version of a summary of the committee's report to Congress by the end of this week.