As the US has acknowledged that the Islamic State group is a greater threat than Al-Qaeda, the White House is reportedly seeking domestic legal justification at home to use unlimited military force against the Sunni extremists in Iraq and Syria.

On Friday, the Obama administration said that the killing of American journalist James Foley by Islamic State (IS, formerly ISIS/ISIL) militants amounts to a direct terrorist attack on the United States.

"We have seen them posing a threat to our interests in the region, to our personnel and facilities in the region, and clearly the brutal execution of Jim Foley represented an affront, an attack - not just on him, he's an American - and we see that as an attack on our country, when one of our own is killed like that," said Deputy National Security Advisor Ben Rhodes.

This comes a day after US Defense Secretary Chuck Hagel stressed that the IS terrorist organization is so well funded and managed, that it “is beyond anything that we've seen. So we must prepare for everything.”

Now the White House is considering seeking domestic approval for military action and a mandate from Congress would do just fine, a senior administration official told the Washington Post.

Congress last time authorized such action over a decade ago, back in 2001 to fight Al-Qaeda after the 9/11 terror attacks, and then in 2002 to topple Iraqi President Saddam Hussein.

The options to defeat IS militarily the US government and Congress are discussing include: obtaining short-term emergency constitutional authority to protect US citizens on territory occupied by the Islamic State; gaining temporary War Powers Resolution authority to defeat Islamists in blitzkrieg military operation; and waging a full-fledged military campaign against IS militants to the bitter end.

It cannot be ruled out that Washington could launch airstrikes or other action in Syria, the official told the WP.

“If you come after Americans, we are going to come after you,” Rhodes said on Friday. “We’re not going to be restricted by borders.”

The US is already using another option it has, a carte blanche for massive air strikes on IS positions throughout Iraq for 60 days, till early October. This has already helped Kurdish and Iraqi forces to take back the key Mosul Dam from Islamic State militants.

But according to the WP source, the Obama administration might go further. Contingency plans for broader airstrikes in Syria have been prepared for presidential review should Obama ask for them, and the intelligence community is trying to identify high-value targets among individual IS leaders.

Meanwhile Washington plans to continue supporting Iraqis and Kurds in their fight with the Islamic State, providing them with arms and training.

According to the official, while the administration is working to formulate a long-term policy, it is currently focused on driving the militants out of Iraq back to Syria, where they were originally fighting the forces of Syrian President Bashar Assad.

Although the US and Arab states of the Persian Gulf have been sponsoring Assad’s adversaries in one of the bloodiest civil wars in the world of recent times, the Islamists that now form the backbone of the IS fighting force have been unable to finish off Assad’s regime, mostly because of the significant popular support the Syrian leader enjoys.

Last year, with Russia’s assistance, President Obama opted not to get the US involved in Syria militarily.

Obama’s administration now has certain legal hurdles to cross before using the existing 2001 Authorization for the Use of Military Force (AUMF) against Al-Qaeda Authority for any military action against IS in Syria. Al-Qaeda announced last year that it had split with ISIS (now the Islamic State), citing its brutality towards Muslims and its declaration of an Islamic caliphate across Syria and Iraq.

“If you look back on the president’s speech, he kind of foreshadows going to have additional AUMFs,” the official said. “It may be that this is the first case.”