|The Islamic Party says merging with al-Shabab will create a stronger force against a weak government [Reuters]
Sheik Mohamed Osman Arus, the Islamic Party's head of operations, told The Associated Press news agency that his militia will join forces with the al-Qaeda linked al-Shabab group and fight under al-Shabab's name.
Fighters from the Islamic Party have been defeated several times by al-Shabab, including a battle to control the port city of Kismayo, one of the largest cities in the country, and now a major base for al-Shabab.
Arus however denied the merger was a face-saving tactic for his group, saying the planned union is "a bonus for the Mujahideen and a bane to the invaders and mercenaries".
He said there will be a formal announcement of the merger soon, but did not give a specific date. Al-Shabab was not available for comment.
Arus said his group's aim to unite with al-Shabab was to influence the hard line elements in it from within, "because any fighting between us will only give more power to the enemy".
Talk of major offensive
The UN-backed Somali government has been promising a major offensive to stamp out al-Shabab for more than a year but the fighters still control three quarters of the country.
The two groups have previously fought together against government forces despite ideological differences and the recent clashes.
The Islamic Party has in the past condemned al-Shabab's use of such tactics as suicide bombers and summary executions.
Sheik Hassan Dahir Aweys, the founder of the Islamic Party, also criticised al-Shabab's public pledging of allegiance to Osama bin Laden.
The Islamic Party has about 2,500-3,000 fighters and was widely seen as having a more nationalist agenda than al-Shabab, which has been heavily influenced by Wahhabi Islam ideology.
Both al-Shabab and the Islamic Party have been plagued by factionalism.
The insurgents hold most of south-central Somalia and much of the capital city, Mogadishu.
So far they have been unable to seize the port, airport or key government installations, which are protected by an African Union peacekeeping force.
Somalia has not had a functioning government for 20 years. Al-Jazeera a