LOKICHOKIO, Kenya (CNN) -- Some of the emergency food supplies meant to help Sudan's famine victims are ending up in the hands of soldiers and rebels, aid officials suspect.

At the U.N.'s aid base in Lokichokio in northern Kenya, a constant stream of wide-bodied cargo planes trundles down the airstrip crammed with sacks of grain to drop over famine-stricken areas.

But as the Herculean operation continues, questions are being asked over whether all the food ends up in the hungry civilian mouths for which it was intended.

The Sudanese government in Khartoum says it believes some food ends up in the hands of rebels from the Sudan's People's Liberation Army (SPLA), against whom it has been fighting a 15-year civil war.

After a visit to southern Sudan last week, the head of the U.N. Children's Fund, Carol Bellamy, said some of the food donated by aid agencies had been stolen by soldiers on both sides.

Using 13 planes -- each making three trips a day into Sudan -- the U.N.'s World Food Program (WFP) hopes to drop 10,000 tons of food in July, at a cost of around $21 million.

WFP employs field monitors to distribute the food to women but says it is difficult to keep track of supplies once the initial distribution takes place.

"From my experience I know it (misappropriation) goes on," said Claude Jibidar, WFP's field coordinator in southern Sudan. "It is not easy to know what goes on at the household level ... There are some elements beyond our control."

In southern Sudan, local people confirmed that some relief goes to feed fighters.

"It ends up with the soldiers indirectly. They see you are cooking it and they ask to have it," said Lazarus Deng, a medical assistant for the Sudan Relief and Rehabilitation Association.

Because of the strong link between famine and war, the issue of misappropriation is a highly sensitive one for aid agencies trying to stem the spread of hunger.

Monsignor Cesare Mazzolari of the Catholic diocese of Rumbek in Bahr el Ghazal said last week as much as 65 percent of food aid in that area is diverted to soldiers.

Jibidar disputed this figure and said that, due to the lack of infrastructure in southern Sudan, it was impossible to pinpoint how much food might be going astray.

Earlier this month, WFP director Catherine Bertini urged the international community to stop the Sudan war.

"We recognize we have an advocacy responsibility on behalf of people ... to blow the bells and whistles to alert others to respond," said Michael Sackett, WFP's eastern Africa regional manager.

"In the meantime we do the best possible job, short-term and Band-Aid though it is."

The United States, which tacitly supports the SPLA in its fight against Sudan's Islamic government, is the largest donor, with $75 million in contributions this year.

Late last year, WFP estimated 250,000 people in southern Sudan would need food aid by spring because of poor harvests.