With the launch of the attack, Godane promised more violence if Kenya refused to withdraw its forces from neighbouring Somalia, where they have been fighting al-Shabaab in its southern heartlands.

“You cannot withstand a war of attrition inside your own country,” he said in an audio message posted on a website linked to al-Shabaab. “So withdraw all your forces, or be prepared for an abundance of blood that will be spilt in your country.”

The slaughter confirms what many in the region have long feared: that Godane, who has imposed a Taliban-style regime in much of war-torn Somalia, would one day begin exporting his brand of Islamist violence to the wider world.

Born in what is now the semi-independent republic of Somaliland, Godane was considered a child prodigy, excelling at Islamic school and winning scholarships to study in Sudan and Pakistan.

As an adult he became an accountant for an airline, but then joined al Itihad al Islamiya, a now-defunct militant group, and went to Afghanistan to fight.

On his return, he and his followers splintered from Itihad’s leadership when it mooted the idea of peaceful politics after September 11, producing the nucleus of what would go on to become al-Shabaab today.

Their first taste of foreign blood was a decade ago, when the same splinter group was responsible for a string of murders of Western aid workers, including Richard and Enid Eyeington, a British couple who ran a popular school in Somaliland.

He has since manoeuvred to become al-Shabaab’s overall commander, although like Mullah Omar, the one-eyed ruler of the Taliban, he is somewhat reclusive − mindful, it seems, of the fate of his comrade Adan Hashi Ayro, who was killed by a US missile strike in 2008.

Tuesday’s American air strikes 105 miles south of Mogadishu were thought to have been designed to achieve the same result.

Under his leadership, during which more moderate rivals have been either killed or sidelined, al-Shabaab has become one of the most brutal militant groups in the world, with stonings and amputations for anyone who defies Godane’s edicts banning music, dancing and even watching football.

It has also advertised Somalia as a base from which to wage global jihad, with Godane using his background in finance and airlines to help recruit hundreds of foreign fighters into the group’s ranks.

However, even its own volunteers faced the group’s wrath if they tried to leave, according to two disaffected ex-Shabaab fighters interviewed for a Panorama documentary to be broadcast on BBC One on Monday night.

The pair, both teenagers from neighbouring Kenya, told how they were horrified to discover that despite

Al-Shabaab’s professed moral piety, its foot soldiers raped and pillaged as much as any other Somali militia. One claimed he saw children as young as five being trained in how to wear suicide vests.

Both teenagers eventually escaped, but hundreds of other volunteers from around the world − including Somalis from Britain − are thought still to be swelling Godane’s ranks.