Islamic State militants are celebrating the capture of a government airbase in northeast Syria, reportedly displaying the severed heads of conquered soldiers as trophies in their stronghold of Raqqa city.

The nearby Tabqa airbase had been President Bashar al-Assad’s last foothold in the Raqqa region, otherwise controlled by IS which has seized large swathes of Syria and Iraq.

The battle for the base is said to have claimed hundreds of lives on both sides.

When it comes to the military might of the group formerly known as ISIS, Syria’s forces believe Washington has questions to answer.

“The expansion of ISIS is not accidental,” said Fawaz Mustafa, head of the Political and Military Academy of the Syrian army.

“More than one country offered a large amount of weapons and funding to it during the civil war. If we look back at the remarks made by senior US officials in Congress recently, we will see that they always knew who is supporting the extremist group in Syria.”

It is another victory for Islamic State, boosted by arms seized in Iraq. Its ranks swelled by fighters from Europe and beyond, it has declared an Islamic Caliphate crossing the Iraq-Syria border.

After the beheading of journalist James Foley, some believe the US should now extend its air strikes from Iraq into Syria – hitting militants where last year it considered targeting President Assad’s forces.