Drought and poor harvests left an estimated 3.6 million people short of food two years ago in the arid country on the southern side of the Sahara.
Shocking images of malnourished children helped launch a global aid appeal.
Unicef's latest nutritional survey showed 15.5 per cent of children below three years of age were acutely malnourished. In some areas the number of cases had risen sharply in recent months.
In two of Niger's eight regions, acute malnutrition was above what is considered the emergency threshold, the fund said.
Critics say aid groups need to revise their strategy if they are to solve the problem in the long term.
Development agencies complain their efforts are undermined by humanitarian relief organisations - who they say bypass local authorities to deliver food aid directly to those in need.
Oxfam and Save the Children were among charities which commissioned a report in June saying foreign organisations were inflexible in aid-solution strategies and needed to bridge the gap between emergency and development responses.
Unicef said it would provide all children under the age of three, outside the capital Niamey, with free supplementary food for two months, in line with the World Food Programme.
The agency will also increase aid to those not covered by feeding centres.
But it said longer-term measures were needed to address "the lack of access to age-appropriate food and feeding practices and the lack of access to basic health services".