"Perhaps the most frightening prospect would be to have a truly megadisaster in a megacity," he said on the first day of a disaster prevention conference in the Japanese city of Kobe, where an earthquake killed nearly 6,500 people a decade ago.

"Then we could have not only a tsunami-style casualty rate as we have seen late last year, but we could see one hundred times that in a worst case."

The five-day conference to mark the 10th anniversary of the Kobe quake is also aiming to draw lessons from last month's quake and tsunami that killed more than 175,000 people along Indian Ocean coastlines.

Megacities have a population of 10 million or more and a dense concentration of people, many of them in slums.

"Time is running short for some of those megacities in Asia, in Africa and in Latin America," Egeland said.

"Some of the megacities are earthquake prone, others are prone to flooding, etcetra. We have to have city planning, we have to have development, we have to have investment in the poor areas, because the poor people now are the most vulnerable," he said.

"There is still time to prevent that, and we hope that some attention could be given to the megacities and not just to the countryside, which we normally associate with tsunamis and with flooding and with drought."

As the world's population continues to grow, so will the size of megacities across the globe, stretching resources and the ability to cope with disasters.

MEGA-GROWTH

According to U.N figures, the top five megacities now are the greater Tokyo area with 35.3 million people, Mexico City with 19 million, New York-Newark 18.5 million, Bombay 18.3 million and Sao Paulo 18.3 million.

But by 2015, the United Nations estimates the populations of the top five will be: the greater Tokyo area at 36.2 million, Bombay 22.6 million, Delhi 21 million, Mexico City 20.6 million and Sao Paulo 20 million.

Tokyo remains a great concern because of its high population, history of earthquakes and impact on the world economy if a major quake devastates the capital of the world's number 2 economy.

Experts say a major quake is long overdue for Tokyo, which was flattened in 1923 by a quake and subsequent fires.

Egeland also said that last month's tsunami, while tragic, could benefit developing nations over time by alerting wealthy donor nations to the importance of spending small sums of money to save lives and property before disaster strikes.

"It has really been a global eye-opener to the devastating impact of natural disasters," he said, adding that he hoped investment would not end when the drama of disaster had faded.

"We have a momentum of understanding, and we have to use that as much as we can to get institutions going and get funds, not only for relief but also for early warning, for prevention and development," he said.