By Daniel Wallis

NAIROBI, April 17 (Reuters) - Poor nations must urgently plan sustainable growth for fast-growing cities, said the United Nations on Tuesday, as hundreds of delegates gathered to discuss urbanisation in Nairobi, home to some of Africa's worst slums.

The United Nations says within months the world's urban population is poised to overtake its rural total for the first time in history. By 2030, it says, two-thirds of humanity will live in cities -- including more than half of all Africans.

The U.N. released a study of the Kenyan capital's environment to coincide with the conference hosted by its Habitat housing agency, highlighting threats to health and development common to scores of cities in poor nations

"This is a sobering assessment of a city in east Africa in the early 21st century facing, as so many urban areas do in developing countries, a significant array of challenges," said Achim Steiner, head of U.N. Environment Programme (UNEP).

Nairobi's population has shot up to around 3 million from 1.1 million in 1985, the report said, but the current plan for its expansion was penned more than 50 years ago.

An estimated 44 percent of residents live in poverty, many of them in sprawling slums without even basic services.

The growing population is spurring fierce competition for land, and even Nairobi National Park -- famed for its lions, leopards, wildebeest and black rhinos -- is threatened, it said.

Poor planning was contributing to traffic congestion, causing regular gridlock and more pollution.

"Our common quest, be it for economic growth, social justice, biodiversity or climate protection will depend to a large and increasing extent on our ability to manage our cities and the urbanisation process," Habitat head Anna Tibaijuka told conference delegates.

The report also highlighted the new phenomenon of urban agriculture, which it said was worth more than $3 million annually in Nairobi and was often carried out by women as a "survival strategy".

"Currently, many low-income farmers are blocking sewers to obtain water for irrigation, increasing the risk of dirty water triggering a rise in the spread of diseases, chemical poisoning and other environmental problems," UNEP said in a statement.

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