US President Barack Obama asked the US Congress on Wednesday to authorise military force against the Islamic State 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 say Obama's foreign policy is too passive and want stronger measures against the militants, also known as ISIL.

With 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 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 several western journalists and aid workers 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.

"I have directed a comprehensive and sustained strategy to degrade and defeat ISIL," Obama wrote in a letter to Congress, calling on lawmakers to back his proposal to "show the world we are united in our resolve to counter the threat posed by ISIL."

Both the US Senate and House of Representatives must approve Obama's plan. Lawmakers said they would begin hearings quickly. Senate Republicans were to meet later on Wednesday.

Republicans criticised Obama's proposal, particularly the limits it sets on using ground troops.

The Republican Speaker of the House of Representatives, John Boehner, told reporters the plan would face hearings and debate "and I'm sure changes," adding: "I'm not sure the strategy that has been outlined will accomplish the mission the president says he wants to accomplish."

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 for failing to seek the backing of Congress, where some accused him of breaching his constitutional authority.

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 White House said Obama would make a statement on his request at 3:30 p.m. (2030 GMT).

'BOOTS ON THE GROUND'

The plan does not authorise "long-term, large-scale ground combat operations" such as those in Iraq and Afghanistan, as the prospect of US 'boots on the ground' once again worries many members of Congress. Obama said those operations would be left to local forces.

But it does allow for certain ground combat operations including rescue operations or the use of special operations forces and it permits the use of US forces for intelligence collection, targeting operations for drone strikes and planning and giving other assistance to local forces.

Democratic Representative Adam Schiff said Obama's proposal contained too few controls. He said a new authorisation should place more specific limits on the use of ground troops and expressed concerns the plan did not set geographic limits.

It was Obama's first formal request for authority to conduct a military operation during his six years in office and, if passed, would be the first war authorisation approved by Congress since lawmakers in 2002 gave then-President George W. Bush authority to wage the Iraq War.

Obama's text includes a repeal of that measure but leaves in place a 2001 authorisation, passed shortly after the September 11 attacks, for a campaign against al Qaeda and affiliates.

Rights groups and many lawmakers have called for the repeal of 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. He said enacting a measure specific to the campaign against Islamic State fighters could serve as a model for revamping the 2001 measure.

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