US PRESIDENT Barack Obama asked Congress today to formally authorise military force against the Islamic State group (Isis), arguing that it could pose a threat to the US homeland.

He urged legislators to “show the world we are united in our resolve to counter the threat.”

The president, who was elected on a promise to end US overseas wars, sent a proposed joint resolution to both houses in the aftermath of the announced death of US hostage Kayla Mueller.

Isis blamed a Jordanian air strike, but Washington insists that the jihadist group put her to death.

In his letter to Congress, Mr Obama said that Isis “poses a threat to the people and stability of Iraq, Syria and the broader Middle East and to US national security.

“It threatens American personnel and facilities located in the region and is responsible for the deaths of US citizens James Foley, Steven Sotloff, Abdul-Rahman Peter Kassig and Kayla Mueller,” he said, listing US hostages who died while in Isis hands.

If left unchecked, he said that Isis would “pose a threat beyond the Middle East, including to the US homeland.”

Mr Obama is offering to limit authorisation to three years, extending to the next president the powers and the debate over renewal for what he envisages as a long-range battle.

He is proposing no geographic limitations to where US forces could pursue Isis and “associated persons or forces,” defined as those fighting on behalf of or alongside Isis “or any closely related successor entity in hostilities against the US or its coalition partners.”

Mr Obama’s proposal bans “enduring offensive combat operations,” which is novel in its ambiguity.

It is clearly designed to bridge the divide between legislators opposed to ground troops and those who say that the commander-in-chief should maintain the option.

His draft would not authorise long-term, large-scale ground combat operations like those deployed in the past in Iraq and Afghanistan.

But it “would provide the flexibility to conduct ground combat operations in other more limited circumstances, such as rescue operations involving US or coalition personnel or the use of special operations forces to take military action,” the president explained.