A senior US senator has accused the CIA of criminal activity in improperly searching a computer network set up for lawmakers investigating allegations of torture.

Democrat Dianne Feinstein, in an extraordinary speech on the Senate floor on Tuesday, publicly aired an intense but formerly quiet dispute between Congress and the spy agency.

The criticism by Feinstein is particularly biting because she has been one of the main defenders of intelligence agencies at a time when they have been sharply criticised at home and abroad following revelations about mass surveillance programmes.

CIA Director John Brennan, asked about Feinstein's accusations, said the agency was not trying to stop the committee's report and that it had not hacked into Senate computers, the Associated Press reported.

He said the appropriate authorities would look at the matter further and "I defer to them to determine whether or not there was any violation of law or principle".

Millions of documents

The CIA provided computers to congressional staffers in a secure room in northern Virginia in 2009, so the panel could review millions of pages of top secret documents in the course of its investigation into the CIA's detentions and interrogations conducted during the Bush administration.

At issue now, is whether the CIA violated an agreement made with the Senate Intelligence Committee about monitoring the panel's use of CIA computers.

The president [Obama] has great confidence in John Brennan and confidence in our intelligence community and in our professionals at the CIA. Jay Carney, White House spokesman

Feinstein said the Senate staff members had an electronic search tool to deal with 6.2 million pages of documents and the ability to make copies on their computers.

She said the arrangement suffered a blow when CIA personnel electronically removed the committee's access to documents that had already been provided to the panel.

She said about 870 documents were removed in February 2010, and an additional 50 were withdrawn without the knowledge of the committee.

Feinstein said the CIA searched the computer network in January and she had pressed Brennan about the agency's actions and the legal basis for its search.

She said she had not received any answers despite letters sent on January 17 and January 23.

Feinstein said the CIA search covered documents as well as "the standalone and walled off committee network drive containing the committee's own internal work product and communications".

Feinstein said the CIA's inspector general, David Buckley, has referred the matter to the Justice Department "given the possibility of a criminal violation by CIA personnel".

An escalating fight

In further evidence of the escalating fight, Feinstein said that after the inspector general's referral, the acting counsel of the CIA filed a criminal report with the Justice Department regarding the committee staff's actions.

Feinstein defended the staff as professionals with appropriate security clearances.

"I view the acting counsel general's referral as a potential effort to intimidate this staff, and I am not taking it lightly," she said.

Democrats on the Senate Intelligence Committee completed their 6,300-page interrogation report last year and are revising it with CIA comments.

Feinstein said she would ask the White House to declassify its 300-plus-page executive summary, and its conclusions.

When the report was first approved by Democrats on the committee, Feinstein said her staffers came to the conclusion that the detention and interrogation programme yielded little or no significant intelligence.

Following the senator's criticism of the CIA, the White House said President Barack Obama has "great confidence" in Brennan.

"The president has great confidence in John Brennan and confidence in our intelligence community and in our professionals at the CIA," White House spokesman Jay Carney told a news briefing.