"We are promising that attacks against the enemy will be routine, more in number and will increase day by day," spokesman Ali Mohamud Rage said in speech broadcast Wednesday by the group's radio.
A suicide bomber on Tuesday detonated an explosives-laden truck at a government compound in Mogadishu, unleashing a powerful blast that damaged buildings and wounded more than 100 others.
The bombing was the deadliest single attack in Somalia by the Shebab, who launched a bloody uprising in 2007 against the Western-backed transitional government.
It was also the first since the insurgents pulled out of Mogadishu in August in a move they said was a change of military tactics.
"The attack was a hit to the mercenaries serving the interests of non-believers who thought that they have captured Mogadishu as well as for those who assume the Shebab had left the capital," Rage said.
"The attack proved that we are still in Mogadishu and very much at K4," said Rage, referring to the area of Mogadishu hit by the suicide bomber.