The explosive growth of urban areas is resulting in greater damage and more deaths from natural disasters than ever before, experts warned Thursday, calling for better planning and safer housing.

Cities now account for half the world's population and are growing faster than their populations can be counted, making them particularly vulnerable to earthquakes, floods and other disasters, especially in poor countries.

"Our investment in risk-reduction preparedness should be wiser. It should not just be chasing the ambulance and responding again and again where something has gone wrong, but investing also in preparedness," said Maggie Stephenson, UN-HABITAT senior technical adviser for Haiti.

Yet even in countries like Japan, home to perhaps the best seismic and earthquake preparedness in the world, preventive measures are sometimes insufficient.

Some 20,000 people died or remain missing there after a huge earthquake and tsunami in March that also caused nuclear reactor meltdowns.

"You can't build your way to safety," warned World Bank urban specialist Abbas Jha.

Speaking with Stephenson and other urban recovery experts at the Brookings Institution, a Washington think-tank, he called for designing disaster mitigation systems that "fail gracefully" and investment in warning systems that are both "credible and timely."

The need is even more urgent, he said, in a world where China will have 223 cities with population greater than one million by 225. Right now, Europe only has 35 such cities. And by 2100, 600 million people will move to vulnerable areas below sea level.

Haiti, already the poorest in the Americas before the quake, still has half a million people living in squalid tent cities nearly two years after a devastating earthquake that killed more than 225,000 people.

Stephenson acknowledged she was "horrified" at the slow pace of reconstruction in Haiti.

Experts agree that it is critical, and cheaper, to train the local population to rebuild rather than have outsiders do it.

"We need to train every single existing mason and every mango-seller that's going to become a mason," Stephenson stressed.

Only 20,000 masons have been trained in Haiti since the earthquake, 10 times less than in northern Pakistan after the massive 2005 quake there, she said.

In the past five years alone, more than 14 million people lost their homes to natural disasters. That can mean more than losing a shelter, said Habitat for Humanity chief executive Jonathan Reckford.

People who work from home can lose their livelihoods, while others lose access to health care, water, sanitation and places of worship, he said.

According to his Christian non-profit group, the number of urban residents worldwide living in areas vulnerable to earthquakes and cyclones will more than double from 680 million people in 2000 to 1.5 billion people by 2050.

"Reconstruction always begins the day after a disaster," he said.

"Our desire and our shared goal is to help families get back to work, back to school, lay that foundation to rebuild their lives."

But rebuilding with future disasters in mind is no easy task in urban areas, home to huge infrastructure issues, land tenure concerns and limited space for reconstruction.