WASHINGTON — Engaging a high-ranking Obama administration official for the first time in an extensive public discussion of the use of drones for targeted killing, members of the Senate Intelligence Committee on Thursday pressed John O. Brennan, President Obama’s nominee for director of the Central Intelligence Agency, about the secrecy of the strikes, their legal basis and the reported backlash they have produced in Pakistan and Yemen.

Adding a new element to the roiling debate, the committee’s chairwoman, Senator Dianne Feinstein, Democrat of California, said she would review proposals to create a court to oversee targeted killings. She gave no details but said such a court would be analogous to the Foreign Intelligence Surveillance Court, which oversees eavesdropping on American soil.

Mr. Brennan was noncommittal, noting that lethal operations are generally the sole responsibility of the executive branch. But he said the administration had “wrestled with” the concept of such a court and called the idea “certainly worthy of discussion.”

On the same day two other administration officials, Defense Secretary Leon E. Panetta, and Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, acknowledged in testimony before the Senate Armed Services Committee that a plan they had supported to arm Syrian rebels had been rejected by the White House, the notably aggressive questioning Mr. Brennan received seemed to underscore the increased scrutiny Mr. Obama’s national security policies are facing as he begins his second term.

Mr. Brennan, 57, a 25-year C.I.A. veteran who has spent the last four years as the administration’s senior counterterrorism official, also faced intense questioning about whether he was responsible for leaks of classified information, and whether he had been candid about his involvement in the agency’s interrogation program under President George W. Bush.

But the senators repeatedly returned to the targeted killings Mr. Brennan has helped direct over the last four years.

The hearing came three days after the leak of a Justice Department document explaining the legal rationale for the killing of American citizens who join Al Qaeda. On the eve of Mr. Brennan’s hearing, the White House gave in to pressure from lawmakers and said the Senate and House Intelligence Committees could see the full classified legal memorandum justifying the killing of Anwar al-Awlaki, an American-born cleric who joined Al Qaeda in Yemen and was killed in Yemen in a C.I.A. drone strike in September 2011.

Ms. Feinstein expressed frustration at the committee’s difficulty in getting information about the targeted killing program. She said that while senators were allowed to view two legal memos, they were still seeking eight others, and committee staff members were still prohibited from reading the classified documents.

In his opening statement, Mr. Brennan acknowledged “widespread debate” about the administration’s counterterrorism operations but strongly defended them, saying the United States remained “at war with Al Qaeda.”

He said later that when C.I.A. drone strikes accidentally kill civilians, those mistakes should be admitted. “We need to acknowledge it publicly,” he said. “In the interests of transparency, I believe the United States government should acknowledge it.”

But senators repeatedly complained that there was too little transparency about the targeted killing program, sometimes producing misleading information in the news media.

“I think that this has gone about as far as it can go as a covert activity,” Ms. Feinstein told reporters after the hearing.

But she defended the agency’s record on the strikes, saying the number of civilians killed each year has been “in the single digits.” A reporter pointed out that she has accused the agency of lying for years about its interrogation program and asked how she could have such confidence in its claims on casualties in the drone program. “I am confident of those figures until I am not confident of them,” she said.

The hearing made clear that even members of the Intelligence Committee, created in the 1970s to be the public’s eyes on secret government programs, are in the dark about many of their details.

Senator Ron Wyden, Democrat of Oregon, told Mr. Brennan that the committee had never been given the full list of countries in which the C.I.A. has carried out lethal operations. Senator Angus King, independent of Maine, urged Mr. Brennan — if he is confirmed, as appeared likely Thursday night — to be more candid with the panel than his predecessors. “There’s no one else watching,” he said.

Even Mr. Brennan had a hard time explaining how much information he thought should be disclosed about targeted killings. “What we need to do is optimize transparency on these issues, but at the same time, optimize secrecy and the protection of our national security,” he said.

The hearing began in chaotic fashion, as protesters began shouting at Mr. Brennan before they were escorted out. One man yelled, “Assassination is against the Constitution!” and one woman held up a sign that read, “Drones Fly Children Die.”

The protests continued as Mr. Brennan began his opening statement. After the fifth interruption, Ms. Feinstein temporarily stopped the hearing and cleared the room, asking that activists from the peace group Code Pink not be readmitted. The rest of the three-and-a-half-hour hearing was held in a mostly empty room.

When Mr. Brennan resumed his testimony, the panel’s top Republican, Senator Saxby Chambliss of Georgia, pressed him on his knowledge of the C.I.A.’s past use of brutal interrogation methods, which were adopted when Mr. Brennan was deputy to the agency’s No. 3 official.

“I had some visibility into some of the activities there,” Mr. Brennan said. “But I was not a part of any type of management structure or aware of most of the details.” He said he opposed coercive methods and expressed objections privately to colleagues.

Mr. Chambliss sounded skeptical, pressing him for details of such conversations, but Mr. Brennan declined to say with whom he raised his objections. Mr. Chambliss also said records showed that Mr. Brennan had received 50 e-mails about the interrogation of Abu Zubaydah, a terrorist facilitator subjected in 2002 to waterboarding, the near-drowning technique then used by C.I.A. interrogators.

Unlike Mr. Panetta, Mr. Obama’s first C.I.A. director, Mr. Brennan declined several times to describe waterboarding as “torture.” He instead called it “reprehensible” and “something that should not be done.”

The committee recently completed a highly critical 6,000-page study of the interrogation program, and Mr. Brennan said the parts of the report that he had read made him question his entire understanding about the program. “I don’t know what the facts are or what the truth is,” he said. He said he wanted to hear the C.I.A.’s response to the report before making a decision about whether it should be declassified.

Some of the most combative moments of the hearing came as Republicans accused Mr. Brennan of disclosing classified information to the press. Senator Jim Risch, Republican of Idaho, accused him of tipping off television commentators about a double agent who foiled a terrorist plot in Yemen to bomb a United States-bound airliner. Mr. Brennan admitted that he told the commentators that the United States had “inside control” of the operation, but denied the suggestion that he had revealed the role of the double agent and said the F.B.I. was investigating the source of that leak.

“It seems to me that the leak the Department of Justice is looking for is right here in front of us,” Mr. Risch said. Mr. Brennan strenuously disagreed, saying he was a witness and not a target of the investigation.

Showing a flash of temper, Mr. Brennan defended his record at the White House. “Senator, I live this every day and night,” he said. “I go to bed at night worrying that I didn’t do enough that day to make sure I protect the American people.”