A contributor for the popular jihadist forum for radical Muslims, Shumuka al-Islam, has threatened comedian David Letterman for  joking about Osama bin Laden's death.

The contributor, who frequently posts comments on the site under the name Umar al-Basrawi, has urged American Muslims to "cut the tongue of this lowly Jew and shut it forever." The posting was reported by the Middle East Media Research Institute, a pro-Israel group that monitors Arab media.

Letterman, a popular American talk show host and comedian, commented on the June 3 killing of al-Qaida leader Ilyas Kashmiri, saying he would join bin Laden, killed a month earlier.

He even "demonstrated the way of the slaughter" by dragging his finger across his neck, the commenter complained.

"Is there not among you a Sayyid Nosair al-Mairi ... to cut the tongue of this lowly Jew and shut it forever?" TIME magazine quoted al-Basrawi writing, Nosair killed Jewish Defense League founder Meir Kahane in 1990.

Letterman is not, in fact, Jewish.

An FBI spokesperson said the bureau is looking into the matter as "we take all of these threats seriously".

Letterman and his publicist declined to comment on the issue.