The United States is leading an international coalition against Islamic State, and President Barack Obama launched an air campaign in August against the militant fighters, who have killed thousands of people while seizing swaths of territory in Iraq and Syria.
The administration's failure so far to seek a formal Authorisation to Use Military Force for the campaign has caused some members of Congress to express concern that it overstepped the president's constitutional authority.
Others have said that lawmakers should weigh in on an issue as important as the use of military force.
The administration has said the campaign is legal, based on authorisation passed under then President George W. Bush in 2002 for the Iraq War and in 2001 for fighting al Qaeda and associated groups.
Nancy Pelosi, leader of the House of Representatives' Democrats, told reporters last week the White House would seek an authorisation that would last three years. She said there had not yet been decisions about the geographic scope of an authorisation or what limits would be placed on combat troops - "boots on the ground."
The issue is expected to be a major sticking point in debate. Many Democrats want to bar sending in U.S. combat forces, but several Republicans have insisted it would be inappropriate to limit military commanders.
The Obama administration had been in close consultations with lawmakers before making its formal request, which could make the approval process move more quickly, he said.
Obama is also expected to seek a repeal of the Iraq war authorization, but not the 2001 authorisation, which passed days after the September 11 attacks.
The White House has declined to comment on the timing or details of the request.

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