A Tunisian suicide bomber named "Abou Abdallah al-Tounissi" boarded the bus in central Tunis on Tuesday and blew himself up, it said in a statement shared on extremist social media accounts.
IS published a photo of the attacker dressed in white and wearing an explosives vest, his head and face covered with a scarf.
At least 11 dead in attack on Tunisia military bus
The militant group said 20 people had been killed, while an earlier official toll said 12 presidential guards died in the attack.
The remains of a 13th person found at the scene - who authorities believe could have been the bomber - have not yet been identified, the interior ministry said.
The bombing was carried out using 10 kilos of powerful Semtex explosive, according to the ministry.
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