The Vatican's ambassador to the United Nations has endorsed military action against the Islamic State group in Iraq and Syria -- an unusual move because the Vatican traditionally has opposed force in the region.

In an interview with the US Catholic website Crux, Archbishop Silvano Tomasi said IS fighters were committing atrocities on a huge scale and the world needed to intervene.

"We have to stop this kind of genocide," the Italian archbishop told Crux. "Otherwise we'll be crying out in the future about why we didn't do something, why we allowed such a terrible tragedy to happen."

Tomasi said a coordinated and "well-thought-out coalition" was needed to do everything possible to achieve a political settlement without violence.

"But if that's not possible, then the use of force will be necessary," he added.