By Steve Holland and Susan Heavey

WASHINGTON, Feb 5 (Reuters) - The White House and its critics faced off on Tuesday over the legality of drone strikes to kill U.S. citizens abroad, in a likely preview of arguments that will be raised during this week's confirmation hearing for President Barack Obama's choice to head the CIA.

The disclosure of an unclassified Justice Department memo laying out the legal framework for the U.S. government's ability to attack its own citizens drew criticism from civil liberties groups. But the White House strongly defended the controversial policy as legal and ethical.

The unclassified memo, first obtained by NBC News, argues that drone strikes are justified under American law if a targeted U.S. citizen had "recently" been involved in "activities" posing a possible threat and provided that there is no evidence suggesting the individual "renounced or abandoned" such activities.

White House spokesman Jay Carney defended current U.S. drone policy, saying they are used to mitigate threats, stop plots, prevent future attacks and save American lives.

"These strikes are legal, they are ethical and they are wise," he said.

Civil liberties groups expressed concerns, while U.S. lawmakers called on the White House to release more of its legal underpinning for the assertion that the president has the power to kill U.S. citizens abroad without trial.

The U.S. government has dramatically increased its use of drone aircraft abroad in recent years to target al Qaeda figures in far-flung places from Pakistan to Yemen.

"My initial reaction is that the paper only underscores the irresponsible extravagance of the government's central claim," Jameel Jaffer of the American Civil Liberties Union wrote on the ACLU's blog. "Even if the Obama administration is convinced of its own fundamental trustworthiness, the power this white paper sets out will be available to every future president."

The use of drones figures to be a prime topic for White House counterterrorism chief John Brennan when he faces the Senate Intelligence Committee on Thursday in a confirmation hearing on his nomination to become CIA director.

The document was disclosed as a bipartisan group of U.S. senators called on the Obama administration to release to Congress "any and all" legal opinions laying out the government's understanding of what legal powers the president has to authorize the killing of American citizens.

The senators who signed the letter, including members of the Senate Intelligence Committee, said the administration's cooperation would "help avoid an unnecessary confrontation that could affect the Senate's consideration of nominees for national security purposes."

One national security official said the leak of the Justice Department memo may have been timed to blunt such congressional demands for the release of additional documents.

Senator Dianne Feinstein, the Democrat who chairs the Intelligence Committee, said in a statement on Tuesday that she had been calling on the administration to release legal analyses related to the use of drones for more than a year.

Feinstein said the document obtained by NBC had been given to congressional committees last June on a confidential basis, and that her committee is seeking additional documents, which are believed to remain classified.

Attorney General Eric Holder on Tuesday said he was concerned that the release of more documents could put sources and operations at risk.

"We'll have to look at this and see how, what it is we want to do with these memos," Holder said.

There is "a real concern to reveal sources, to potentially reveal sources and methods and put at risk the very mechanisms that we use to try to keep people safe, which is our primary responsibility," he said at a news conference.

'IMMINENT THREAT'

In the unclassified Justice Department paper, the authors laid out three conditions that the executive branch should meet before a drone strike is ordered.

A top U.S. official must determine that the targeted person "poses an imminent threat of violent attack against the United States," cannot be captured, and that the strike "would be conducted in a manner consistent with applicable law of war principles," the department said.

The memo is drawing new attention to the 2011 strike that killed U.S.-born Anwar al-Awlaki, who U.S. investigators say was a leader of al Qaeda's Yemen-based affiliate and linked to a botched plot to blow up a U.S. airliner with a bomb hidden in a man's underwear on Christmas Day 2009.

Targeted killings carried out by remotely piloted unmanned aircraft are controversial because of the risks to nearby civilians and because of their increasing frequency. The United Nations recently launched an investigation into their use.

Most such attacks have been carried out by the United States, but Britain and Israel have also used drones.

Hina Shamsi of the ACLU, which has sued for more information on the drone program, called the memo "profoundly disturbing" and "a stunning overreach of executive authority."

Shamsi, head of the ACLU's National Security Project, in a statement called on the Obama administration to release what she said was a 50-page classified legal document on which the 16-page summary is based.

"Among other things, we need to know if the limits the executive purports to impose on its killing authority are as loosely defined as in this summary, because if they are, they ultimately mean little," she said late Monday.

The ACLU on Tuesday will also file court papers seeking to block government efforts to dismiss the group's lawsuit challenging the 2011 killing of Awlaki and two other Americans in Yemen, the statement said. 

(Additional reporting by David Ingram, Mark Hosenball and Patricia Zengerle)

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