WASHINGTON  — A Justice Department document 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 confidential memo, 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 that delaying action against individuals continually planning to kill Americans would create an unacceptably high risk.

It adds that the threat posed by al-Qaida and its associated forces demands a broader concept of when a person continually planning terror attacks presents an imminent threat.