Islamic State threatened to burn alive seventeen Kurdish fighters kept in cages and paraded around on streets, as shown in a recently posted video of the militants.

Shackled and dressed in orange jumpsuits, the caged fighters in the video are believed to be in Iraq's northwestern city of Kirkuk.

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On Friday, several Islamic State fighters who launched an offensive against an Iraqi air force station that is home to around 400 US forces were struck down in a counter-offensive by Iraqi forces, a US military spokesman said.

Iraqi forces killed eight Islamic state fighters in and around a gate to al-Asad air base in Iraq's western province of Anbar, said Pentagon spokesman Colonel Steve Warren.

Warren ruled out U.S. troops' involvement in the counter-offensive, adding the troops were stationed around two miles away in a different area of the big air base.

He said American Apache attack helicopters and surveillance drones were sent to the site of the attack but they did not take part in the fighting.

Meanwhile, U.S. intelligence officials warned air strikes against militants in Syria and Iraq did little to stop the growing number of foreign fighters joining Islamic State and other militant groups.

In an updated estimate, the numbers count some 3,400 extremist followers from Western nations among the 20,000 from around the globe.

Homeland intelligence reports said around 150 Americans have attempted while some have succeeded in making it to the Syrian war zone. Some of the followers were captured while on their way while others died in the area and several took up arms with the militants.

The director of the U.S. National Counter Terrorism Center, Nick Rasmussen, said the rise in the number of foreign fighters traveling to Syria is unmatched in history -- far beyond the rate of foreigners who answered the call to wage jihad in Somalia, Yemen, Iraq, Afghanistan and Pakistan in the past two decades.

In the meantime, the White House has come up with a proposal for Congress to authorize US forces in clamping down on Islamic State over the next three years. An official request for legislation is expected over the weekend.