The US government had placed a USD 5 million bounty on the head of spy chief Mahad Karate, with Kenya celebrating what it called a "major blow" to the Al-Qaeda-linked group.
Kenyan troops, part of an African Union force in Somalia (AMISOM) fighting the insurgents, claimed to have killed Karate, a top commander also responsible for internal security, in an air strike at a Shebab training camp earlier this month.
Karate -- head of the Shebab's Amniyat unit, a special security wing responsible for intelligence, attacks and assassinations -- is said to have been involved in plotting the 2015 massacre of 148 people at Garissa university in northeastern Kenya.
"The Kenya Defence Forces, under AMISOM operations, would like to confirm that Mahad Mohammed Karate... And 10 other middle level commanders were killed in a major KDF strike," in southern Somalia on February 8, Obonyo said.
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The claims could not be independently verified, and there was no immediate reaction from the Shebab.
But the 10-day delay in announcing Karate's death suggests thorough efforts to identify the remains were carried out.
Karate was at a Shebab training camp to "preside over the passing out of an estimated 80 Amniyat recruits who had completed their training, and were due for deployment to carry out more terrorist attacks," Obonyo said.
Karate was put on the US State Department's wanted terrorist list in April 2015 after the Garissa attack, which followed the 2013 attack in the Kenyan capital when Shebab gunmen slaughtered at least 67 people at Nairobi's Westgate Mall.
"Karate, also known as Abdirahman Mohamed Warsame, played a key role in the Amniyat, the wing of the Shebab responsible for assassinations and the April 2, 2015 attack on Garissa University College," said the US Rewards for Justice wanted notice.
"The Shebab's intelligence wing is involved in the execution of suicide attacks and assassinations in Somalia, Kenya and other countries in the region, and provides logistics and support for Shebab terrorist operations throughout the Horn of Africa," it added.