Niger is now facing the worst hunger crisis in its history, the UN's World Food Programme says, with almost half the population - or 7.3 million people - in desperate need of food.
A WFP spokesman said villagers in Niger described the situation as worse than 2005, when thousands died of hunger.
After a prolonged drought, heavy rains have now hit parts of the country, killing at least six people.
The WFP says 17% of children, or one in five, are acutely malnourished.
The figure - based on national surveys in May and June - is well above the WFP's normal 15% threshold for declaring an emergency.
It is appealing for $213m (£136m) in aid, but is still 40% short of its target, a spokesman said.
The charity Helen Keller International (HKI) has accused the international community of failing to respond effectively to repeated appeals for help for Niger.
HKI's Africa director, Shawn Baker, told the BBC that tens of thousands of children would die unless more aid is pledged.
"Famine is a very loaded word," he said, "but I think if you look in terms of the number of children affected, the way the livestock have been decimated, and the population movements that were seen earlier in the year, you certainly could consider a famine situation."
He said the government of Niger was doing all it could.
The UN said more than 67,000 people lost their homes after severe rains in the past week.
The River Niger - the third largest in Africa - reached its highest level for 80 years, said the regional river authority, the ABN.
But the rains came too late to rescue this year's crops, which have already failed.
"This year was a double whammy," Christy Collins of the aid agency Mercy Corps told the Associated Press news agency.
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