NEWPORT, United Kingdom (AFP) - U.S. President Barack Obama  outlined plans on Friday for a broad international coalition to defeat Islamic State extremists in Iraq and Syria, but they received a cautious response from European allies.

Speaking at a NATO summit, Obama said the United States would also try to involve Middle Eastern allies in a strategy to counter the jihadists, who have overrun large swathes of territory.

"It's not going to happen overnight but we are steadily moving in the right direction and we're going to achieve our goal," Obama said.

The president said regional involvement was "absolutely critical", although the State Department added that there were "no plans" for military coordination with Iran in the fight.

"We're going to degrade and ultimately defeat ISIL," Obama added, using the previous name of the Islamic State, which has claimed responsibility for videos showing the beheading of two U.S. journalists.

The executions appear to have been carried out by the same British-accented militant, who also threatened to kill a U.K. hostage unless American air strikes in the regions are halted.

Iraqi Kurdish forces and Shiite militias on Friday discovered mass graves containing 35 bodies after retaking the town of Sulaiman Bek from jihadists.

Along with Iraqi military troops, they broke a months-long jihadist siege of the town of Amerli last Sunday and retook Sulaiman Bek the following day in the first major successes for the federal government since the crisis began.