She has pipe-stem limbs and displays every rib on her narrow chest, but two-year-old Hasana is not sick enough to be treated in a hospital.

Under a white plastic tent, an aid agency doctor has a few minutes to make decisions about the lives of scores of babies.

Outside his tent, a sea of desperate mothers queue in the boiling sun, hoping for food for their children.

While aid is beginning to arrive in Niger, the numbers of children needing treatment is still rising.

In the last week of July, more than 1,600 children were admitted to treatment programmes run by the medical charity Médecins Sans Frontières in two of the worst-affected regions.

The seeds of the crisis were sown when locusts and drought destroyed crops and pasture land for cattle in parts of Niger.

But it was economics, not the hand of nature, which led to the crisis spiralling out of control. The price of the staple food, millet, has soared, because traders have been exporting grain from unaffected parts of Niger to wealthier west African countries such as Nigeria.

While markets here are filled with produce, the poorest starve because they cannot afford the prices.

"I collect firewood to sell, so I can buy food at the market," said Mallia, sitting outside the aid agency tent with Hasana, her daughter. "But I have to carry five loads of wood to the market to make enough [money] for a cup of millet."

As the crisis grew, the government of Niger at first refused to distribute free food, fearing that it would disrupt the market. But that policy has now been reversed and the first general distribution of free food will begin today.

Hasana is clearly in a desperate state. Lying amid the folds of her mother's yellow-green robes, she weighs 3.5kg. A child of her age should normally weigh 5.1kg.

Her twin sister, Huseina, died at the end of last month. Her mother has walked for a day to seek help, and her flip-flops are caked with dirt. But the doctor decides that Hasana is not sick enough to be hospitalised. The infant is given a taste of a nutritional supplement, a sweet-tasting paste that resembles peanut butter. The medical staff note that she is eating well; a sign that she may be able to recover by herself. Child and mother are sent home with several sachets of the supplement, as well as powdered milk and oil.

Johanne Sekkenes, head of the mission in Niger for MSF-France, said: "I would say that 60% of [the] children will never be admitted into the feeding centre.

"They are still severely malnourished. They will still die if we don't take care of them. But they are, if I may say so, well enough to stay at home and come once a week [to the mobile clinic], because the mothers are the best caretakers."

In the nearby town of Tahoua, baby Moussa is one of those deemed sick enough to be brought into an intensive care unit.

Lying on a narrow bed, milk has to be fed to him through a tube in his nose. The slender, naked boy with a slightly puffed belly cannot swallow anything because of a fungal infection in his mouth.

Moussa's skin is dotted with sores from another infection. Ganda Amadou, a Nigerian doctor at the intensive care unit, said: "Malnutrition is followed by low immunity.

"The child is not eating, and if he wasn't here he would die - because how can his mother feed him?"

In Yama, a village of elaborately sculpted mud houses, hundreds of mothers have brought their children to seek help as two aid agencies, MSF and Concern, work side by side.

While the medical charity deals with the severely malnourished children, women with children who are slightly less desperate are given food aid by the other charity.

In the sweltering afternoon heat, tensions begin to build. The mothers at the front of the queue to receive food are those whose babies have been identified as malnourished by teams who went out the week before. Each of these babies wears a bracelet to mark them out.

Behind them is a press of equally desperate women whose children have not been marked for priority treatment.

As all the "braceleted" babies receive food and pass through, anxiety builds among the mothers at the rear of the queue that they will receive nothing.

There is a surge, and the ropes around Concern's tent are knocked down. Swiftly, local elders help to restore order and urge the women back, but the aid workers fear a stampede if they begin the distribution again. Instead, the food is packed away, and driven to local warehouses in a fleet of four-wheel-drive vehicles.

It is an illustration of the challenges faced by the charities delivering aid here.

Maureen Crill, a nurse working for Concern, said: "There was over 1,000 people, and if they started pushing in, we would be asking for people to be crushed, especially babies.

"I know how horrible it looks. It was a tough decision to make."