The Washington Post reports that six Islamic scholars have issued a fatwa, or religious opinion, saying that Muslims serving in the U.S. military must defend their country, even in combat against other Muslims.

The fatwa affirms that Muslims must perform the obligations of citizenship, including military service.

Part of the opinion reads "All Muslims ought to be united against all those who terrorize the innocents, and those who permit the killing of non-combatants without a justifiable reason."

It was signed by a Qatar-based Islamic figure, Sheik Yusuf Qaradawi, and five others. The other signers are Taha Jabir Alwani, an Islamic teacher based in Virginia, three Egyptian scholars, and a resident of Syria. Several of the signers have been critical of U.S. foreign policy in the past, particularly as regards Israel.

The Pentagon says there are about 4,000 Muslims serving in the military. But the scholars say the number may be three times that.

The opinion was published on September 27 in response to an inquiry by a U.S. Army Muslim chaplain, Captain Abdul-Rasheed Muhammad. The Post describes him as the first Muslim chaplain ever appointed in the U.S. armed forces.