UNITED NATIONS, July 6 (Reuters) - The United Nations appointed Malian diplomat Cheick Sidi Diarra on Friday as its top envoy for the world's poorest countries and for small islands, many of them environmentally threatened.

Diarra, 50, who is currently Mali's ambassador to the world body, succeeds Anwarul Chowdhury of Bangladesh as High Representative for the Least Developed Countries, Landlocked Developing Countries and Small Island Developing States.

The United Nations classifies 50 states as "least developed countries," 34 of them in Africa. The definition is based on criteria including per capita income, economic vulnerability, health, education and adult literacy levels.

Chowdhury, who had held the post for five years, played a prominent role after the devastating December 2004 Indian Ocean tsunami in efforts to strengthen the defenses of small islands vulnerable to climate change and rising sea levels.

Diarra has been a lead negotiator on African integration issues at African summits and was also involved in his own West African nation's internal reconciliation efforts after the 1991 overthrow of autocrat Moussa Traore, a U.N. statement said.

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