Nairobi — As Al-Shabaab intensified its onslaught on Mogadishu all week, it paraded four teenagers in a public square whom it announced will suffer cross amputation -- right hand and left foot cut off -- in accordance with Islamic sharia law.

The plight of the teenagers, who were accused of stealing a pistol and three cellphones, signalled the continuing struggle between the militant Islamist insurgents and the transitional Somalia government they want to overthrow over the scope and severity of the sharia law the former have vowed to implement.

When in February newly-elected President Sheikh Sharif Ahmed (himself a moderate Islamist) caved in to rebel demands for Islamic law, he desperately needed to blunt the edge off the Islamist insurgents whose campaigns of law and order were striking a popular chord, especially in southern Somalia.

The Islamists other loud objection to Africa Union peacekeepers being deployed from non-Islamic countries -- Uganda and Burundi -- had also considerably boxed in President Ahmed.

Sheikh Ahmed's tactical concession has won him no respite as it has increasingly become clear that his government and the insurgents have radically different interpretation of sharia.

Though on paper the beleaguered transitional government acceded to the pressure for Islamic law, it has in practice been reluctant to implement a strict Taliban-style version, such as forbidding girls from schooling, decreeing mandatory veils for women, or banning music and television.

But in the months Al-Shabaab and its Hezbul-Islam allies have been running southern Somalia, the insurgents have demonstrated that their interpretation of Islamic law is far more extreme. In Kismayu, they have carried out public amputations and floggings of accused criminals.

There was also a reported case early in the year of a couple stoned for alleged adultery.

The latest decision on the four Mogadishu teenagers, which Amnesty International has unreservedly condemned, has sent a chilling message on what awaits those perceived as wrongdoers in the besieged capital.

"The fight for legitimacy is no longer just about who will win the military battle, but whose interpretation of sharia will carry the day," says Muqtar Hersi, an independent Islamic scholar in Mogadishu.

Al-Shabaab have staked their campaign not just over Islamic purity but also on strict law and order. And compared to lawless Mogadishu, the order they have imposed in places they control like Kismayu has made many weary Somalis give them more than the benefit of the doubt.

Still, the likelihood of a popular backlash is high. Somalis are by tradition Sunni moderates. The former Islamic Courts Union (ICU) had also briefly introduced sharia in Mogadishu and the southern region before they got ousted by invading Ethiopians in 2006.

But there was soon an uproar against the harsher tenets of the law, the last straw being the ICU's banning of khat (miraa). It also did not help matters that the ICU chose to ban TV during the month the 2006 World Cup was being played in Germany.