The Vatican's envoy to the United Nations in Geneva has endorsed military action against the Islamic State group in Iraq and Syria.
The church has traditionally 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 said.
"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."
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Tomasi said a co-ordinated 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.
Pope Francis has denounced the "intolerable brutality" being inflicted on Christians and other minorities in Iraq and Syria by IS group militants.
Last month, IS kidnapped 220 Assyrians in the Tal Tamr area of Syria where the extremist Islamist group has seized control of 10 Christian villages, according to the Syrian Observatory for Human Rights.
The Pope also expressed his dismay after the group's Libyan branch released a video showing the gruesome beheading of 21 mostly Egyptian Coptic Christians.
