Niger's government cannot afford to distribute free food to those hit by a food crisis, officials say, as hundreds of people flee to Nigeria.
Some 150,000 children are malnourished, aid agencies say.
Food crisis committee manager Seydou Bakary warned of a "nationwide catastrophe" if this year's harvests are even slightly deficient.
Children are dying as a result of the famine brought about by drought and a locust plague, the United Nations says.
Donors have largely ignored a UN appeal to help the 3.5m people going hungry.
Two children are dying each day in one feeding station in Maradi, 600km east of the capital, Niamey, UN World Food Programme spokesman Marcus Prior told the BBC.
Some 150,000 young children are severely malnourished after poor rains and locust invasions devastated last year's harvest.
Nigerian immigration officials say there has been a "substantial" increase in the number of people crossing from Niger.
"They are fleeing from the famine facing them," said Hassan Suleiman Kangiwa, head of the Nigerian immigration service in northern Katsina State.
He said that security had been increased at border posts and that those without valid documents would be sent home.
Mr Bakary, however, told the AFP news agency that the issue was being "politicised" and that people always went hungry in one of the world's poorest countries.
"We should be cautious not to exaggerate the situation - there is chronic malnutrition throughout the country, even during the most productive harvests."
He did say that in some parts of Niger, people are only eating once a day and have started to eat wild plants because nothing else is available.
"It is almost impossible to identify with certainty the most vulnerable families in an area plagued by poor crops and food insecurity, which is why we will avoid free distribution of food until the situation demands it," he said.
There have been protests in the capital, Niamey, of people who accuse the government of ignoring the problems.