Niger's President Mamadou Tanja has visited the country's south, where severe food shortages are affecting at least 2.5 million people.
His government has been defending its handling of the crisis, saying its appeals for international assistance in November went unanswered.
The UN says 150,000 children could die following last year's disastrous crop.
The charity Oxfam said families were feeding their children grass and leaves from trees to keep them alive.
A government official told the BBC: "We have made an appeal since November and told the international community... We did not have any response."
Critics accuse Niger of being slow to call for help compared to other countries in the region which also experienced poor rains and plagues of locusts eating their crops.
Malnourished people are arriving every day at the few feeding centres in the region.
But the BBC's Hilary Andersson in southern Niger says fewer than one in 10 of the starving make it to the centres.
Top United Nations aid official Jan Egeland on Wednesday accused the international community of reacting slowly to the crisis in Niger.
The crisis was widely predicted after last year's poor harvests, following poor rains and locust invasions.
"Niger is the example of a neglected emergency, where early warnings went unheeded," Mr Egeland told the BBC.
In June, the Niger government refused demands to distribute free food and has been criticised for not doing more to prepare for the food shortages.
"The world wakes up when we see images on the TV and when we see children dying," Mr Egeland told the BBC's World Today programme.
"We have received more pledges in the past week than we have in six months. But it is too late for some of these children."
The slow response has greatly increased the cost of dealing with the crisis, aid workers say.
"The funding needs are sky-rocketing because it's a matter of saving lives," UN World Food Programme Niger representative Gian Carlo Cirri said.
"The pity is we designed a preventative strategy early enough, but we didn't have the chance to implement it."
Aid workers in Niger say that up to a quarter of Niger's 12 million people need food aid.
The UN has now received just a third of the $30m it had asked for, Mr Egeland said.
The UN under-secretary general for humanitarian affairs also said that beyond immediate food aid, the world should help Niger improve its agricultural methods to avoid future food crises - but this programme had received even fewer pledges.
He said the $30m requested for both short - and long-term aid "was nothing".
"Europeans eat ice cream for $10bn a year and Americans spend $35bn on their pets each year."