A Muslim militant has urged American followers to assassinate talk show host David Letterman, saying his tongue deserved to be cut out, a U.S. intelligence monitoring group said on Wednesday.

The SITE intelligence group said the death threat was posted on an online website used by militants after the writer became upset by a joke Letterman made about the death of a leading member of al Qaeda killed in an air strike in Pakistan.

The militant called on Muslims in the United States to "cut the tongue of this lowly Jew and shut it forever," the threat against Letterman said, according to a translation by SITE. The popular late-night television host is not Jewish.

The FBI said it was taking the threat seriously.

The writer was angered by watching Letterman on his show drawing his finger across his neck while talking about the June death of senior al Qaeda figure Ilyas Kashmiri.

The message was posted on the website Shumukh al-Islam. SITE analyst Adam Raisman told Entertainment Weekly the website was "a clearing house for al Qaeda material" and drew supporters of the group headed by the late Osama bin Laden.

Executives at broadcaster CBS and a spokesman for Letterman declined on Wednesday to comment on the threat.

Tim Flannelly, an FBI spokesman in New York, said the agency was looking into the matter.

"The FBI takes all threats seriously and we will run it down like we do all threats," Flannelly said.

A Muslim group in 2010 threatened the makers of "South Park" after the animated satirical show depicted the Prophet Mohammad in a bear suit. Most Muslims consider any depiction of the founder of Islam as offensive.

A 21-year old man from Virginia was sentenced in February to 25 years in prison for publishing the home addresses of the "South Park" creators and urging readers to pay them a visit, as well as trying to help a Somali militant group.