A U.S.-based intelligence monitoring group says a Muslim militant has urged his Islamic followers to kill U.S. talk show host David Letterman.

The SITE Monitoring Service said the death threat was posted Tuesday on a Website used by Muslim extremists, after the militant became upset by a joke Letterman made on his show about the death of an al-Qaida leader, Ilyas Kashmiri.

Kashmiri was killed in a U.S. drone strike in Pakistan in June.

The militant called on Muslims in the United States to — as he put it — “cut the tongue off of this lowly Jew and shut it forever.”

Letterman, a popular late-night talk show host for the U.S. network CBS, is not Jewish.

The FBI said it is looking into the threat.