US President Barack Obama has asked Congress to authorizse military force against Islamic State (IS) that would bar any large-scale invasion by US ground troops and limit operations to three years.
Republicans, who control Congress, put up swift resistance to the proposal. They said Obama's foreign policy is too passive and want stronger measures against the militants.
With Mr Obama's fellow Democrats wary of another Middle East war, it could be difficult for the White House to win enough support to pass the bill, even though six months have passed since the military campaign began.
Some lawmakers predicted a vote as soon as March but others anticipated debate could last for months.
The proposed resolution said Islamic State, "has committed despicable acts of violence and mass execution."
Its militants have killed thousands of civilians while seizing territory in Iraq and Syria in an attempt to establish a hub of jihadism in the heart of the Arab world.
They have also generated international outrage by beheading several western journalists and aid workers and burning to death a Jordanian pilot.
Mr Obama sent his request to Congress a day after his administration confirmed the death of Kayla Mueller, a 26-year-old aid worker who was the last known American hostage held by the group.
Both the US Senate and House of Representatives must approve Mr Obama's plan.
The plan does not authorize long-term, large-scale ground combat operations such as those in Iraq and Afghanistan, as the prospect of ground forces worries many members of Congress.
It was Mr Obama's first formal request for authority to conduct a military operation during his six years in office.