But senators pressed Kerry, Defence Secretary Ashton Carter and top US military officer Martin Dempsey on whether a new resolution would lead to "mission creep" that could see hundreds of US ground troops sent back into the Middle East.

"Our nation is strongest when we act together -- and we simply cannot allow this collection of murderers and thugs to achieve its ambitions," Kerry told the Senate Foreign Relations Committee.

The Islamic State (IS) group, which has captured a large swath of territory in Iraq and Syria, wanted to ensure "the death or submission of all who oppose it" as well as "the incitement of terrorist acts across the globe," he said.

Kerry called for a united vote in favor of a new authorization for use of military force (AUMF) to give Obama "a clear mandate to prosecute armed conflict against ISIL and associated persons or forces."

Tantamount to a declaration of war, the authority would provide Obama political cover at home and a firmer legal basis on which to prosecute the fight, particularly among allies abroad who have joined a US-led international coalition.

So far, the Obama administration has used the existing authorization approved in the days after the September 11, 2001 attacks to go after Al-Qaeda militants and the Taliban as justification for the war on IS, also known as ISIL.

"The president already has statutory authority to act against ISIL, but a clear and formal expression of your backing would dispel any doubt anywhere that Americans are united in this effort," Kerry said.

The top US diplomat also insisted that the "administration sees no need for US forces to engage in enduring offensive ground combat operations against ISIL."
    
Obama took office in 2009 vowing to end the wars in Iraq and Afghanistan, amid battle fatigue among Americans. But he has already sent military advisors back to Iraq to combat IS, and is weighing whether to slow down the withdrawal of US forces from Afghanistan.

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