Humanitarian agencies paid Somalia’s al-Shabab militants for access to areas under their control in the 2011 famine, according to a joint report by two think tanks.
In many cases al-Shabab insisted on distributing the aid and kept much of it for itself, the report says.
Some of the groups are still paying al-Shabab to operate in the large parts of Somalia it still holds, it adds.
More than 250,000 people died during the 2011 famine.
The report – by the Overseas Development Institute and the Mogadishu-based Heritage Institute for Policy Studies – details how al-Shabab demanded from the agencies what it described as “registration fees” of up to $10,000 (£6,100).
It gives one example of al-Shabab diverting food aid in the town of Baidoa, where it is reported to have kept between half and two-thirds of food aid for its fighters.
Al-Shabab developed a highly sophisticated system of monitoring and co-opting the aid agencies, even setting up a “Humanitarian Coordination Office”.
Aid groups had to deal with this office, even though they risked legal problems by doing so because of counter-terrorism laws in other states which forbid engagement with groups like al-Shabab.
The report says agencies who worked in al-Shabab-held areas had to complete special forms and sign a pledge saying they would refrain from certain social and religious activities.
It also describes how al-Shabab gave people extra food if they spied on the aid groups.
Some agencies were banned outright by al-Shabab, while others withdrew because of the demands.