A senior US senator on Tuesday accused the Central Intelligence Agency of illegally searching computers of Senate staff members who were investigating a CIA interrogation program.

Dianne Feinstein, the powerful chairwoman of the Senate Intelligence Committee, angrily denounced the CIA's actions, saying it appeared to be a bid to intimidate lawmakers from holding the spy agency accountable.

"I have grave concerns that the CIA's search may well have violated the separation of powers principles embodied in the United States Constitution," Feinstein said in a dramatic speech on the Senate floor.

The CIA searched the computer drive used by staffers on the intelligence committee who were preparing an elaborate report examining the agency's controversial and now defunct interrogation program, she said.

"I have asked for an apology and a recognition that this CIA search of computers... was inappropriate," she said.

"I have received neither."

Feinstein said she and the vice chairman of the intelligence committee learned of the search on January 15 in an "emergency meeting" requested by CIA director John Brennan.

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," she said.

Feinstein's extraordinary speech was a major break from her usually cordial relations with the intelligence community, which she has often defended against accusations of overstepping its authority.

The senator's comments came after unnamed administration officials alleged to news media that senate staffers took sensitive documents without authority, triggering an investigation.

Analysts say the rift between Congress and the CIA over the case is the worst since the 1970s, when lawmakers uncovered illegal abuses and introduced legal reforms to rein in the spy services.