(Reuters) - The CIA conceded on Thursday that it had improperly monitored computers used by the U.S. Senate Intelligence Committee in an investigation of CIA interrogation tactics and secret prisons for terrorism suspects after the Sept. 11, 2001 attacks. Central Intelligence Agency spokesman Dean Boyd said in a statement that the agency's inspector general had determined that "some CIA employees acted in a manner inconsistent" with an understanding between the agency and the Senate panel.

Boyd said CIA Director John Brennan had informed Sen. Dianne Feinstein, the Senate committee's chair, and its senior Republican, Sen. Saxby Chambliss, of the finding and apologized.

The Senate committee has been investigating excesses allegedly committed by CIA officers who used harsh interrogation methods, including waterboarding or simulated drowning, and established a network of secret prisons abroad.

Human rights activists and critics of the CIA's methods, including some U.S. politicians, have described the CIA's interrogation methods as torture.

The White House is expected to deliver a declassified summary of the committee's report, and the CIA and Republican responses, to Congress by the end of this week.

Feinstein will have to decide whether an apology is sufficient or if some further action is needed, Steven Aftergood, a national security expert at the Federation of American Scientists said.

Senator Mark Udall, a Democrat on the intelligence committee, said that the Inspector General's report had undermined his support for Brennan.

"From the unprecedented hacking of congressional staff computers and continued leaks undermining the Senate Intelligence Committee's investigation of the CIA's detention and interrogation program to his abject failure to acknowledge any wrongdoing by the agency, I have lost confidence in John Brennan," Udall said in a statement.

Officials familiar with the committee's report say it concludes that the use of coercive interrogations did not produce any significant counter-terrorism breakthrough in the years after the 2001 al Qaeda attacks.

According to the officials, the report says CIA officials misstated or exaggerated the results of the program to other agencies and to Congress by claiming that the harsh methods had helped to foil terrorist plots.

The CIA and Intelligence Committee Republicans, whose participation in the panel's investigation was limited, have written critiques which officials said dispute the conclusions of the Senate committee.