"Today, eight out of the world's ten most populous cities are prone to earthquakes, and six of them are on or near the coast. A billion people live in unstable, overcrowded slums," said John Holmes, the UN under-secretary general for humanitarian affairs.

"The combination of decaying infrastructure, land erosion, crowded conditions, and a lack of rescue services could lead to catastrophes on an unprecedented scale," said Holmes on the first day of a UN disaster reduction conference in Geneva.

He told journalists that governments, international organisations, civil society and the private sector all had a responsibility to put disaster risk reduction strategies at the heart of policy-making.

"It's not enough to set up a national disaster management centre unless you give it some resources, but also ... you have to 'mainstream' this so that disaster risk reduction is built into policies of all different kinds."

This should apply as much to World Bank and United Nations programmes as to those of national governments, Holmes added.

Salvano Briceno, director of the UN's International Strategy for Disaster Reduction secretariat, said huge "mega-cities" in the developing world -- such as Mumbai, Mexico City, Bogota, Istanbul, Jakarta, Shanghai and Manila -- could all be at risk. "Most of them are prone to either heavy storms, heavy cyclones, earthquakes and sometimes several of the hazards combined," he said.

"There's more people living in the wrong places and doing the wrong things," he added, citing people living on "risky" land that is either close to the sea or prone to erosion and landslides.

Holmes said climate change meant that phenomena such as earthquakes could become more severe as sea levels rise, thus heightening the risk of a tsunami striking coastal areas.

Moreover, global warming coupled with existing environmental degradation could give rise to new threats such as forest fires and mudslides.

"The risks and vulnerabilites are very clear, the need to something about them is very clear, and that need has been very considerably exacerbated by what is happening on the climate change front," Holmes said.