U.S. fighter in Somalia says threatened by fellow militants NAIROBI (Reuters) - An influential American fighter for Somalia's al Shabaab rebels, who has in the past urged Western-backed Muslims to join the group, has dismayed radical Islamists by saying his life is under threat from fellow guerrillas due to internal disputes. Experts on Somalia said the video by Omar Hammami suggested splits over ideology and strategy were weakening al Shabaab, which joined al Qaeda in February and is fighting to topple Somalia's weak interim government. Such a public disavowal by a serving member of an al Qaeda aligned militant organization is highly unusual, and will be seen as a significant public relations setback for the movement. In a video uploaded on Friday, Hammami, who goes by the name Abu Mansour al-Amriki, says: "To whomever it may reach from the Muslims, I am Abu Mansour al-Amriki. I record this message today because I feel that my life may be endangered by Harakat al-Shabaab al-Mujahideen due to some differences that occurred between us regarding matters of the Sharia (Islamic law) and matters of strategy. Hammami is an American from Alabama who has in the past uploaded hip-hop chants taunting the United States and urging it to make him a martyr in a drone attack. He is a familiar online figure among Western-based communities of Islamist radicals that support al Shabaab. In a statement on Twitter, al Shabaab on Saturday said it was "surprised" by the video, a formal investigation was underway and the group was still attempting to verify the authenticity as well as the motivations behind the video. "We assure our Muslim brothers that al-Amriki is not endangered by the Mujahideen and our brother still enjoys all the privileges of brotherhood," it added, using a term that refers generally to Islamic guerrilla groups or holy warriors. A Kenyan Islamist group, the Muslim Youth Centre (MYC), which is a vocal online supporter of al Shabaab, issued several messages on its Twitter account on Saturday that appeared to be a response to Hammami's video. They did not refer to the American, but referred to "rumors or facts of al Shabaab's disunity among senior members" and called on "our brothers in al Shabaab to demonstrate unity of purpose" by carrying out more attacks such as one on March 14 in which a suicide bomber killed four people onside the presidential palace compound in Mogadishu. Counter-terrorism experts suggested that the Hammami video would have a demoralizing effect on al Shabaab's foreign fighters, believed to number several hundred including some U.S. and other Western nationals, because it indicated that there were divisions near the top of the organization. One independent expert on al Shabaab said that Hammami was seen as selfish by some inside the organization and was known for attention seeking activity. Another expert, Evan Kohlmann, a terrorism consultant in the United States, said: "Hammami has a long history of classic narcissistic behavior, and before he joined al-Shabaab, he was notorious for getting into nasty online arguments with other Muslim extremists over rather silly points of contention. He has a very high opinion of himself and his knowledge of Islam, and frankly, I'm not entirely surprised to hear that al-Shabaab might have grown tired of his constant pedantry. Ben Venzke of Intelcenter, a US-based terrorism monitoring group, said it was unprecedented in recent history for a member "of a major terrorist group to release a video fearing for his life from the very group he joined." He has played a significant role in recruiting new fighters to al-Shabaab and is believed to have been on track for a growing role and profile not only in the group but in the broader jihadi community. The mere release of the video, not to mention if al-Shabaab kills him, is a damaging blow to the jihadists that he inspired to go to Somalia and join al-Shabaab. It is unclear if this development has anything to do with the group's formal joining of al-Qaeda. Editing by James Macharia and Andrew Heavens