By Abdi Sheikh and Feisal Omar

MOGADISHU (Reuters) - A leader of Somali Islamist group al Shabaab, who has a $3 million U.S. bounty on his head, has surrendered, a Somali government official and local media said on Saturday, although the militants said he had long left their organisation.

Zakariya Ismail Ahmed Hersi was one of seven al Shabaab leaders for whom the U.S. State Department in 2012 offered a total of $33 million in reward money for information that led to their capture.

If true, Hersi's surrender would be the second major blow to al Shabaab's leadership in just a few months. In September the group's main leader, Ahmed Abdi Godane, was killed by a U.S. drone strike.

"Al Shabaab leader Zakariya Ismail surrendered to government forces in El Wak, Gedo region. He is expected to be flown to Mogadishu tomorrow," a senior government official told Reuters.

State radio website Radio Muqdisho also reported Hersi's surrender, describing him as "the general secretary of al Shabaab's finance (department)". It did not give reasons for his surrender.

But a senior member of al Shabaab's media team said Hersi left the Islamist group two years ago.

"The government exaggerates the story just to cover the recent attack at the AU base," the al Shabaab official told Reuters by phone, referring to this week's attack on an African Union base in the capital Mogadishu.

"(Hersi) cannot have impact on al Shabaab because he is not a member."

The government offered an amnesty to al Shabaab members in September, but key leaders have not taken up the offer.

The al Qaeda-aligned group wants to topple the Western-backed Mogadishu government and impose its own strict version of Islamic law in the country.

Although al Shabaab still controls chunks of the countryside in south and central Somalia, this year it has lost several key towns during two major offensives by African Union peacekeepers and the Somali National Army.

However, the group continues to carry out hit-and-run attacks in Somalia and in neighbouring Kenya, where it has killed hundreds of people over the past 18 months.

(Writing by Drazen Jorgic; Editing by Alison Williams and Raissa Kasolowsky)