Somalia's al-Shabab militants in Mogadishu say three members of another Islamist insurgent group, Hizbul Islam, accidentally blew themselves up while planting a roadside bomb that was meant to kill al-Shabab officials.  The explosion comes on the heels of an announcement by Hizbul Islam that it has begun fighting al-Shabab in the Lower Juba region.

Al-Shabab officials showed local journalists the bodies of three men they say were killed while planting an improvised explosive device in Bakara, a large open-air market area in Mogadishu under the control of al-Shabab.  

Al-Shabab says the men were members of Hizbul Islam, the fundamentalist-nationalist insurgent group that formed an alliance with al-Shabab in early 2009 to fight Somalia's U.N.-backed government and the 5,300 African Union peacekeepers protecting the government.

Residents in Bakara market confirm hearing a loud explosion early Monday.  There has been no comment from Hizbul Islam officials.

A day earlier, Hizbul Islam's information secretary, Mohamed Moallim, announced the alliance between Hizbul Islam and al-Shabab in Somalia was over.

The Hizbul Islam official says fighting between the two Islamist groups has begun because al-Shabab is refusing to peacefully resolve what Moallim describes as "misunderstandings."

One of the biggest disputes erupted early this month when Hassan Turki, a prominent Islamist leader of a faction of Hizbul Islam called the Ras Kamboni Brigade, joined al-Shabab's ultra-conservative Islamist movement.  Al-Shabab, a self-declared ally of al-Qaida, then claimed that all members of Ras Kamboni had joined al-Shabab.

The declaration enraged Ras Kamboni's clan-based military leadership, including Hassan Turki's brother-in-law Ahmed Madobe.  Since September, Madobe and his forces have been locked in a power struggle with al-Shabab in the Lower Juba region.  The two sides have fought fierce battles in the port city of Kismayo and the Kenyan-Somali border town of Dobley, among others.   

Tensions between the Islamist groups also rose after the leader of Hizbul Islam, Sheik Hassan Dahir Aweys, sharply criticized al-Shabab for violating an agreement to not poach leaders or fighters from each other.

On Sunday, Hizbul Islam's information secretary hinted the insurgent group is now rallying behind Ahmed Madobe, stating that Hizbul Islam, not just Madobe's faction, had fought al-Shabab last week for control of Dobley.   After several days of violence, Dobley, which has changed hands several times in recent months, is reportedly under al-Shabab control again.

Meanwhile, in the Somali capital Mogadishu, businesses have shut down and residents are leaving before what many believe will be a major clash between Hizbul Islam and al-Shabab in the coming days.  Hundreds of residents had already fled Mogadishu this month in anticipation of a long-awaited government offensive against al-Shabab.

Just recently, residents had reported that truckloads of Hizbul Islam fighters had positioned themselves on the outskirts of Mogadishu to support al-Shabab in the fight.  Neither Hizbul Islam nor the Somali government has indicated whether those fighters will now fight on the same side.