Jakarta is just one of Asia's megacities - defined as home to more than 10 million people - a growing list of urban jungles that place great strain on local governments and the millions of people who flock from rural areas seeking jobs.

The United Nations predicts that by 2015 there will be 27 megacities in the world, with 15 in Asia alone. That compares with only 12 in 1990. Of that number, seven were in Asia.

From Jakarta to Mumbai, and further afield from Lagos to Sao Paulo, life can be a constant battle with sickness, crime and communal unrest.

The U.N. hopes the Johannesburg meeting can make a difference, but experts believe it will have its work cut out.

World leaders at the summit are expected to pledge to "significantly improve" by 2020 the lives of 100 million of the world's slum dwellers, many of whom live in megacities like Jakarta, capital of Indonesia which is hosting two weeks of pre-summit talks on the resort island of Bali.

Among the biggest concerns are sickness.

"There is a very big health issue. Respiratory disorders for example are massive in many of these cities and the public health costs are very high," Robert Wasson, director of the Centre for Resource and Environmental Studies at Australian National University in Canberra, told Reuters in Bali.

Through an action plan being negotiated at a luxury hotel complex in Bali, Johannesburg also aims to help the poor by boosting access to clean water and modern energy supplies.

The slum dwellers' initiative envisages actions to improve access to land and property, adequate shelter, along with credit programmes for the urban poor. It is not clear who will pay.

Wasson said he believed the quality of life in megacities would probably get worse because poor countries could not, or would not, invest in the infrastructure needed for a clean up.

Indeed, whether Asia liked it or not, the future was more megacities, said Shobhakar Dhakal, a researcher at the Institute for Global Environmental Strategies in Japan.

"It's very difficult to stop megacities growing, particularly in Asia. The Asian way of organisation is a little different than Europe, with the trend being large concentrations of people in certain locations," said Dhakal, also speaking in Bali.

The Asian rate of urbanisation was also much faster than the rest of the world, added Dhakal, an expert on megacities.

Both Wasson and Dhakal said the U.N. target would be hard to reach, considering the funding and the political will required on the part of local authorities to take action.

But Wasson said there was also some benefits to megacities.

"You can concentrate problems. With lots of people, their waste and energy consumption, if you manage it sensibly and in a sustainable way then you can have a huge impact," he said.

He said that in two Asian megacities, Beijing and New Delhi efforts were being made to control air pollution. In New Delhi, he said that was being driven by people, not government.

Much of the prodding to improve megacities would come from growing middle classes, such as in New Delhi, Wasson added.