Washington - President Barack Obama's plan to install his counterterrorism adviser as director of the CIA has opened the administration to new scrutiny over the targeted-killing policies it has fought to keep hidden from the public.

The administration's refusal to provide details about one of the most controversial aspects of its drone campaign - strikes on U.S. citizens abroad - has emerged as a potential source of opposition to CIA nominee John Brennan, who faces a Senate confirmation hearing scheduled for Thursday.

The secrecy surrounding that policy was punctured Monday with the disclosure of a Justice Department "white paper" that spells out the administration's case for killing Americans accused of being al-Qaida operatives.

The timing of the leak appeared aimed at intensifying pressure on the White House to disclose more-detailed legal memos that the paper summarizes - and at putting Brennan, Obama's top counterterrorism adviser, on the defensive for his appearance on Capitol Hill.

Administration officials on Tuesday sought to play down the significance of the disclosure, saying that they have already described the principles outlined in the document in a series of speeches.

"One of the things I want to make sure that everybody understands is that our primary concern is to keep the American people safe, but to do so in a way that's consistent with our laws and consistent with our values," Attorney General Eric Holder said in response to questions about the paper.

Nevertheless, the leak and signals from senior lawmakers that they may seek to delay, if not derail, Brennan's confirmation made it clear that Obama's decision to nominate him has drawn the White House into a fight it had sought to avoid.

Sen. Ron Wyden, D-Ore., a member of the intelligence committee, said Brennan's level of influence and the timing of his nomination have given lawmakers leverage that they lacked in previous efforts to seek details from the White House.

Brennan "is the architect of the administration's counterterrorism policy," Wyden said. "If the Congress doesn't get answers to these questions now, it's going to be extremely difficult, if not impossible, to get them in the future."

The white paper, which was first reported by NBC News, concludes that the United States lawfully may kill one of its own citizens overseas if it determines that the person is a "senior, operational leader" of al-Qaida or an affiliate and poses an imminent threat.

But the 16-page document allows for an elastic interpretation of those concepts and does not require that the target be involved in a specific plot, because al-Qaida is "continually involved in planning terrorist attacks against the United States."

The paper does not spell out who might qualify as an "informed, high-level official" able to determine whether an American overseas is a legitimate target. It avoids specifics on a range of issues, including the level of evidence required for an American to be considered a "senior, operational" figure in al-Qaida.

The document's emphasis on those two words, which appear together 16 times, helps to explain the careful phrasing the administration employed in the single case in which it intentionally killed an American citizen in a counterterrorism strike.

Within hours of Anwar al-Awlaki's death in September 2011, White House officials described the U.S.-born cleric as "chief of external operations" for al-Qaida's affiliate in Yemen, a designation they had not used publicly before the strike.

Officials said that Awlaki, previously portrayed mainly as a propagandist, was directly involved in a series of plots, including the attempted bombing of a Detroit-bound airliner on Christmas 2009.

The white paper, which was distributed confidentially to certain lawmakers last summer, doesn't indicate when the underlying Justice Department memos on targeted killings of Americans were completed.

As a result, it is unclear whether the memos were in place before first apparent attempt to kill Awlaki, a joint U.S.-Yemeni strike shortly before the foiled Detroit plot in 2009.

Three other Americans have been killed in U.S. airstrikes in Yemen since 2002, including Awlaki's 16-year-old son. But U.S. officials have said they were casualties of attacks aimed at other senior al-Qaida operatives.

Civil liberties groups described the white paper as an example of the kind of unchecked executive power that Obama campaigned against.

"The parallels to the Bush administration torture memos are chilling," said Vincent Warren, executive director of the Center for Constitutional Rights. Warren accused Obama of hypocrisy for ordering Bush administration memos released publicly while maintaining secrecy around his own. To deliver on his promises of transparency, Warren said Obama "must release his own legal memos and not just a Cliffs Notes version."

White House spokesman Jay Carney emphasized that the white paper is unclassified and indicated the administration does not intend to release the classified legal memo on which it is based. Asked whether Obama would respond to demands from lawmakers that he release the original document, Carney said "I just have nothing for you on alleged memos regarding potentially classified matters."

The number of attacks on Americans is minuscule compared to the broader toll of the drone campaign, which has killed more than 3,000 militants and civilians in hundreds of strikes in Pakistan, Yemen and Somalia.

The administration frequently has described its domestic and international legal rationales for drone strikes in general terms. The white paper expands those justifications with specific determinations to be made in the case of U.S. citizens.

The struggle between the administration and Congress is relatively narrow, limited mainly to the White House's refusal to turn over a collection of classified memos rather than any broad-scale opposition to the use of drone strikes or even the killing of Americans.

Most members of Congress agree with administration assertions that the drone campaign has been essential to crippling al-Qaida and its ability to mount large-scale attacks against the United States.

Sen. Dianne Feinstein, D-Calif., the chairman of the intelligence committee that will consider Brennan's nomination, released a statement Tuesday indicating that she believes the release of the white paper - which was apparently done without the consent of the administration - should quell calls for more transparency.

The administration's legal position "is now public and the American people can review and judge the legality of these operations," Feinstein said. She has indicated she will support Brennan's nomination.

Brennan, 57, has presided over a major expansion of the drone campaign, although he is also credited with imposing more rigorous internal reviews on the selection of targets. He spent 25 years at the agency and was considered a likely candidate for the CIA job in Obama's first term. He withdrew amid mounting opposition from civil liberties groups that called attention to his role as a senior CIA executive when the agency began using interrogation techniques, including waterboarding, that subsequently were denounced as akin to torture.

Washington Post staff writer Julie Tate contributed to this story.