By Abdoulaye Massalatchi

NIAMEY, July 31 (Reuters) - More than one in seven children under 3 years old in Niger are suffering from acute malnutrition as the annual lean season begins, despite massive aid since a 2005 food crisis, the U.N. Children's Fund said on Tuesday.

Drought and disastrous harvests left an estimated 3.6 million people short of food two years ago in the landlocked, arid country on the southern side of the Sahara. Shocking images of malnourished children helped prompt a global aid appeal.

UNICEF said that while the number of children suffering from the severest form of acute malnutrition had halved since 2005, thanks in part to humanitarian aid, many were still in danger.

"At the onset of the lean season, when communities are most vulnerable, still too many children are at risk of malnutrition," the fund said in a statement.

UNICEF's latest nutritional survey showed 15.5 percent of children under 3 in the former French colony, one of the poorest nations on earth, 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.

The arid regions around Diffa in the remote east and Agadez in the north, the gateway to the Sahara, were particularly hard hit, while the situation was critical around Maradi and Zinder in the south, considered as prime farming areas.

Malnutrition rates are permanently at emergency levels in many parts of the Sahel, the band of savannah stretching across the southern Sahara from Senegal to Chad.

Critics say aid groups need to rethink their strategy if they are to solve the problem in the long term.

Development agencies complain their efforts to work with government institutions are undermined by humanitarian relief organisations, who they say bypass local authorities to deliver food aid directly to those in need.

A report commissioned by charities including Oxfam and Save the Children said in June foreign organisations were inflexible with preconceived aid solutions for the region and needed to bridge the gap between emergency and development responses.

UNICEF said it would provide all children under 3 outside the capital Niamey with free supplementary food for two months, in coordination with the World Food Programme, and increase aid to those not already covered by feeding centres.

But it also 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".

"The situation is compounded by the lack of access for women and caregivers to life-saving information, education and support within a context of massive and pervasive poverty," it said.

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