Rome, Italy - The hijacking of United Nations aid deliveries by Islamic State of Iraq and the Levant (ISIL) fighters in Syria sparked outrage this week, reviving a debate about how humanitarian groups should work effectively to ensure crucial supplies reach victims in conflict zones.

Photos of fighters handing out what appear to be boxes of aid from the UN World Food Programme (WFP) with "Islamic State in Syria" labels pasted over the WFP logo have been circulating on the internet.

From al-Shabab in Somalia to illegal armed groups in Colombia, the problem of these groups commandeering food assistance is not new, but it is a sensitive topic for charities behind aid deliveries.

The WFP condemned the "manipulation of desperately needed food aid". The group is trying to confirm the authenticity of the images but acknowledged that food from a warehouse near Aleppo, Syria, was stolen last September.

"We believe this is an isolated incident," WFP spokeswoman Abeer Etefa said.

Security analysts and academics studying the problem say charities need to be more open when discussing where aid is going, how much disappears, and who benefits. Aid groups should stop suppressing information about theft for fear of alienating donors, observers said.

Sometimes aid workers are forced to give food to fighters to be able to work in an area, sometimes armed groups expropriate and sell it, and sometimes armed factions take the aid directly to the intended recipients. Nancy Qian, Yale University Funding for relief is a constant problem with the WFP suspending food aid for more than 1.7 million Syrian refugees in December after facing a shortfall of $64 million, which donors eventually raised.

"What goes on in the field is not transparent ... but if we just recognise a lot of aid is being misappropriated, then maybe aid agencies will be more likely to share data," said Yale University economist Nancy Qian, who studies aid flows.

"Sometimes aid workers are forced to give food to fighters to be able to work in an area, sometimes armed groups expropriate and sell it, and sometimes armed factions take the aid directly to the intended recipients."

In some cases agencies might not have data about lost or stolen supplies because of logistical constraints of operating in conflict zones, Qian said.

Humanitarian groups tend to focus on how much aid they give out rather than its effectiveness, and transparency could lead to better targeting of aid, she said.

Undisclosed losses

Several large charities, including Oxfam and Save the Children, refused interview requests on how armed groups can co-opt or steal aid, underlining the sensitivity of the subject for brand-conscious organisations constantly seeking funding.

"We do know about this issue," a spokesman for Oxfam said in an email, but "given the sensitivities" of the situation in Syria and Iraq "we have to decline your interview".

Save the Children did not respond to emails and a representative of the group reached by telephone said they would not be commenting.

Information about the theft of WFP food rations last September was shared with the group's major donors, Etefa said, which are mostly governments. The information was not released publicly.

The food, enough to feed up to 8,000 people for a month, was to be distributed by WFP partner groups, including the Syrian Arab Red Crescent and other local organisations, Etefa said.

"There are challenges, risks involved in doing humanitarian work in conflict areas, but that doesn't mean we give up," she said.

"Nothing justifies pulling out [of Syria or Iraq]."

At least 200,000 people have died and half the Syrian population of 22 million has been displaced by the conflict that began with anti-government protests in 2011.

Richard Barrett, a veteran British diplomat now with the Soufan Group, an intelligence firm, said the reluctance of aid groups to speak about the long-running problem isn't surprising.

"This is a bad thing for aid agencies in all respects," Barrett said, adding donors were less likely to contribute if they believed aid was going to ISIL.

The financial state of ISIL Barrett said the crude attempts to rebrand the WFP aid boxes were unlikely to fool anyone but the action could embolden ISIS, that controls swathes of territory in Syria and Iraq, in its efforts to portray its "caliphate" as a genuine state.

As part of its quest for the legitimacy of a functioning government, ISIL has tried to manage the entire food supply in regions under its control, including aid, by seizing grain silos and even regulating prices for street food in some areas.

An exodus of farmers and a lack of seeds and other farm input means the food situation in ISIL-held regions is expected to worsen after the next harvest, Barrett said, potentially increasing pressure on militants to pilfer aid.

But cutting off assistance wouldn't be a smart strategy, from a military or humanitarian perspective, he said.

"Islamic State soldiers will be the last people to suffer from food shortages. The population will suffer first," Barrett said.

A version of this story first appeared on the Thompson Reuters Foundation news service