US president Barack Obama on Wednesday sent Congress his long-awaited formal request to authorise military force against Islamic State, meeting swift resistance from Republicans as well as his fellow Democrats wary of another war in the Middle East.

Republicans, who control Congress and say Obama's foreign policy is too passive, want stronger measures against the militants than outlined in the plan, which bars any large-scale invasion by US ground troops and covers the next three years.

Obama acknowledged that the military campaign is difficult and will remain so. "But our coalition is on the offensive. ISIL is on the defensive, and ISIL is going to lose," he said in a televised statement from the White House.

With many of Obama's fellow Democrats insisting the plan is too broad because it includes no blanket ban on ground troops, it could be difficult for the authorisation to pass, even though six months have passed since the campaign began.

Obama consulted with Republicans and Democrats in writing the resolution, and said he would continue to do so. He said the time frame was intended to let Congress revisit the issue when the next president takes office in 2017.

The proposal says 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 western aid workers and journalists and burning to death a Jordanian pilot.

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 Senate and House of Representatives must approve Obama's plan. Lawmakers said they would begin hearings quickly as Republicans made clear they thought the plan fell short.

The Republican Speaker of the House of Representatives, John Boehner, told reporters he was sure the plan would change as it moved though Congress. "I'm not sure the strategy that has been outlined will accomplish the mission the president says he wants to accomplish," he added.

Obama has defended his authority to lead an international coalition against Islamic State since August 8 when US fighter jets began attacks in Iraq. The formal request eased criticism of Obama's failure to seek the backing of Congress, where some accused him of breaching his constitutional authority.

SEEKING A UNITED FRONT

With Republicans in control of Congress after routing Obama's Democrats in November elections, the president also wants lawmakers to share responsibility for the campaign against Islamic State and present a united front.

The plan does not authorise "long-term, large-scale ground combat operations" such as those in Iraq and Afghanistan.

Obama said those operations would be left to local forces, but lawmakers worried they would not step up. "What is the role, really, that regional partners are playing in this battle against ISIL?" asked Democratic Senator Tim Kaine.

The draft allows for certain ground combat operations including hostage rescues and the use of special forces. It permits the use of US forces for intelligence collection, targeting operations for drone strikes and planning and giving other assistance to local forces.

Many Democrats, especially liberals in the House, said Obama's proposal was too broad. They want any authorisation to place stricter limits on the use of ground troops and expressed concerns Obama set no geographic limits on the campaign.

"The language ... is very broad, very ambiguous," said Democratic Representative Adam Schiff. "None of us really know what 'enduring offensive combat operations' means."

It was the first formal request for authority to conduct a military operation of Obama's six years in office. If passed, it would be Congress' first war authorisation since then-President George W. Bush's 2002 authority to wage the Iraq War.

Obama's objection as a US senator to that authority helped fuel his successful 2008 campaign for the White House.

Obama's text includes a repeal of the 2002 Authorization for the Use of Military Force. But it leaves in place an open-ended authorisation, passed days after the September 11, 2001, attacks, for a campaign against al Qaeda and its affiliates.

Rights groups and many lawmakers said they want the new AUMF to set an end date for the 2001 authorisation, which the White House has invoked to carry out drone and missile strikes against suspected al Qaeda militants in Yemen and Somalia.

Obama said he remained committed to working with Congress to "refine, and ultimately repeal" it.