WASHINGTON—A confidential Justice Department memo says it is legal for the government to kill U.S. citizens abroad if it believes they are senior al-Qaida leaders continually engaged in operations aimed at killing Americans.

The document, first reported Monday night by NBC News, provides a legal rationale behind the Obama administration's use of drone strikes against al-Qaida suspects.

The 16-page document says it is lawful to target al-Qaida linked U.S. citizens if they pose an "imminent" threat of violent attack against Americans, 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-Qaida 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 added.

A September 2011 drone strike in Yemen killed Anwar al-Awlaki and Samir Khan, both U.S. citizens linked to al-Qaida.

The memo does not require the U.S. to have information about a specific imminent attack against the U.S.

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

The document also says that a decision maker must take into account that "the U.S. government may not be aware of all al-Qaida 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."