Hammami's messages have included hip-hop chants taunting the United States to make him a martyr in a drone attack.

Hammami, known by the nickname Abu Mansour al-Amriki, is one of only a few Americans on the FBI's Most Wanted Terrorist List, including Oregon-born Adam Yahiye Gadahn, who was added to the list in 2006.

Gadahn, a convert to Islam who has appeared in al Qaeda videos wearing robes and a turban and warning the United States that it would face attacks if it did not heed militant demands, was added to the list after a California grand jury indicted him for treason. Gadahn, born Adam Pearlman, remains at large.

The FBI said in a statement that Hammami traveled to Somalia in 2006 and joined al Shabaab.

The Somali group, which was designated by the U.S. State Department as a foreign terrorist organization in 2006, wants to topple the Somali government and impose a strict interpretation of Islamic law across the Horn of Africa state.

It was unclear whether Hammami's inclusion on the list was prompted by an indictment in the United States. An FBI supervisor assigned to the case declined to comment beyond the information in the bureau's press release.

Earlier this year, in a video that experts said at the time suggested splits over ideology and strategy were weakening al Shabaab, Hammami said in a video that his life was under threat from fellow militants due to internal disputes.

In addition to Hammami, the FBI added Raddulan Sahiron, a native of the Philippines, to the Most Wanted Terrorists List on Wednesday. According to the FBI, Sahiron is wanted for his alleged involvement in the 1993 kidnapping of an unnamed American by the Abu Sayyaf insurgent group.