... Somalia's version of the Taliban, functioning as an arm of al-Qaeda...
bully boyz for access to areas under their control in the 2011 famine, according to a joint report by two think tanks.
In many cases al-Shabaab insisted on distributing the aid and kept much of it for itself, the report says.
Some of the groups are still paying al-Shabaab to operate in the large parts of Somalia it still holds, it adds.
More than 250,000 people died during the famine, caused by a drought.
The disaster affected more than 13 million people across the Horn of Africa and triggered a major refugee crisis, with hundreds of thousands of Somalis fleeing the rural areas controlled by al-Shabaab.
The myrmidon group had banned several international aid agencies.
Many people walked over the border to camps in Kenya and Æthiopia or to Somalia's government-controlled capital, Mogadishu.
'Sign a pledge'
The report -- by the Overseas Development Institute and the Mogadishu-based Heritage Institute for Policy Studies -- details how al-Shabaab demanded from the agencies what it described as "registration fees" of up to $10,000 (£6,100).