The CIA conceded on Thursday that it had improperly monitored computers used by the US Senate Intelligence Committee in an investigation of 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 Senator Dianne Feinstein, the committee's chairwoman, and its senior Republican, Saxby Chambliss, of the finding and apologized.

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

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

According to an unclassified summary of the inspector general's report obtained by Reuters, five agency employees, two lawyers and three information technology staffers, "improperly accessed" a data network Senate investigators were using to pursue their inquiry.

The summary said the CIA's Office of Security also looked at how Senate investigators accessed the data network and conducted a "keyword search of all and review of some" of the investigators' emails sent through the network.

As tension built between the CIA and the committee this year, the agency asked the Justice Department to open a criminal investigation into whether committee staffers used the network to access privileged CIA information.

However, the inspector general's summary said it turned out that the "factual basis" for the criminal referral the agency sent to the Justice Department "was not supported" because the lawyer making the referral "had been provided inaccurate information."

White House press secretary Josh Earnest said Brennan had "done what is necessary to get to the bottom of what had happened."

The CIA said Brennan had ordered a further inquiry, headed by former senator Evan Bayh, to see if disciplinary actions or institutional reforms were needed.

Senator Mark Udall, a Democrat on the intelligence committee, went further, calling on Brennan to resign and saying the activity demonstrated "a tremendous failure of leadership."

But there was no such call from the committee leaders.

Feinstein declined to comment. Chambliss noted that Brennan had kept him and Feinstein informed, telling reporters: "Until I find otherwise, or unless he fails to hold individuals accountable that breached the Senate computer, I'm going to withhold judgment."

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.

Officials familiar with the report said it concludes that the use of coercive interrogations did not produce any significant counter-terrorism breakthrough in the years after the 2001 attacks and that CIA officials misstated or exaggerated the results to other agencies and to Congress.