The US government's internal guidelines for targeted killings of al-Qaeda suspects allow for such strikes against US citizens abroad, as long as they are believed to be senior leaders of the group and still engaged in operations, a leaked justice department memo shows.

The 16-page document, released by the US-based NBC news television service on Tuesday, provides a legal rationale behind the US administration's use of drone strikes against al-Qaeda suspects.

The memo says that it is lawful for the US to target al-Qaeda-linked US citizens if they pose an "imminent" threat of violent attack against other US citizens, and that delaying action against such people would create an unacceptably high risk.

Such circumstances may necessitate expanding the concept of "imminent threat", the memo says.

"The threat posed by al-Qaeda and its associated forces demands a broader concept of imminence in judging when a person continually planning terror attacks presents an imminent threat," the document says.

Specific imminent threat

The memo said that authorities did not have to possess information regarding a specific imminent attack against the US.

It does require that the capture of terrorism suspects be first deemed unfeasible, and that any such lethal operation by the US targeting a person comply with fundamental law-of-war principles.

"A decision maker determining whether an al-Qaeda operational leader presents an imminent threat of violent attack against the United States must take into account that certain members of al-Qaeda [...] are continually plotting attacks against the United States" and that "al-Qaeda would engage in such attacks regularly to the extent it were able to do so", the document says.

The document also says that a decision maker must take into account that "the US government may not be aware of all al-Qaeda plots as they are developing and thus cannot be confident that none is about to occur; and that [...] the nation may have a limited window of opportunity within which to strike in a manner that both has a high likelihood of success and reduces the probability of American casualties".

With this understanding, the document said, a high-level official could conclude, for example, that an individual poses an imminent threat of violent attack against the US where he is an operational leader of al-Qaeda or an associated force.

'Profoundly disturbing'

The American Civil Liberties Union (ACLU) said the document is "profoundly disturbing".

"According to the white paper, the government has the authority to carry out targeted killings of US citizens without presenting evidence to a judge before the fact or after, and indeed without even acknowledging to the courts or to the public that the authority has been exercised," Jameel Jaffer, ACLU's deputy legal director, wrote on the organisation's website.

"Without saying so explicitly, the government claims the authority to kill American terrorism suspects in secret."

He termed the limits set out in the memo to be "so vague and elastic that they will be easily manipulated".

The document says that the use of lethal force would not violate the Fourth Amendment of the US constitution when a targeted person is an operational leader of an enemy force and an informed, high-level government official has determined that he poses an imminent threat.

The document said the courts have no role to play in the matter.

"Under the circumstances described in this paper, there exists no appropriate judicial forum to evaluate these constitutional considerations. It is well established that 'matters intimately related to foreign policy, and national security are rarely proper subjects for judicial intervention'," the white paper said.

Reporting from Washington DC, Al Jazeera's Patty Culhane said that the Obama administration had consistently failed to share the details of the legal frameworks under which such strikes are ordered.

"It's been a consistent and unanswered question from the Obama administration," she said. "The memo that apparently gives them the legal justification [to carry out drone strikes in general] they won't even share with Congress - this [leaked memo] is a softer, broader memo."

Awlaki case

In of the most prominent cases of US citizens being killed abroad by the government, a drone strike in Yemen in September 2011 killed Anwar al-Awlaki and Samir Khan. Awlaki was a high-level al-Qaeda operative, while Khan edited al-Qaeda's English language magazine.

Neither had ever been formally charged with a crime.

The US operates armed drones in several theatres of operation, most notably Yemen and Pakistan.

NBC said the leaked memo was given to the Senate Intelligence and Judiciary Committees in June on the condition that it be kept confidential and not discussed publicly.

Its leak comes just two days before John Brennan, the White House counterterrorism chief, goes before the Senate for hearings on his nomination to be head of the CIA.

Brennan has been a central player in the US drone campaign, which has expanded sharply under President Barack Obama despite qualms about its legality and public outrage in Pakistan over civilian deaths.