WASHINGTON (REUTERS) - The White House will ask the US Congress for new authority to use force against Islamic State in Iraq and Syria (ISIS) by Wednesday, congressional aides said on Monday.

The United States is leading an international coalition against ISIS, and President Barack Obama launched an air campaign in August against its extremists in Iraq and Syria.

But proposed language the administration will send to lawmakers this week would be the first time the administration will ask for a formal Authorisation to Use Military Force for the fight.

The delay has caused some members of Congress to express concern that the campaign against the militant group overstepped the president's constitutional authority.

The administration has said the campaign was legal, based on authorisation passed under President George W. Bush in 2002 for the Iraq War and in 2001 for fighting al Qaeda and associated groups.

Nancy Pelosi, the House Democratic leader, told reporters last week the White House would seek an authorization 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" - for the fight against ISIS militants.

Obama is also expected to seek a repeal of the Iraq war authorisation, but not the 2001 authorization.

Aides said on Monday that was still the expectation for Obama's request, given discussions between the administration, members of congress and congressional staffers.

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