GENEVA, May 2 -- The Gaza Strip is filled with more war debris and rubble than Ukraine, the head of UN demining operations for the narrow Palestinian territory said Wednesday.

And the danger for clearance work is restricted not just to unexploded ordnance but includes possible exposure to toxic substances such as asbestos.

The United Nations Mine Action Service (UNMAS) estimated the amount of debris in Gaza at 37 million tons in mid-April, or 300 kilogrammes per square meter.

"Gaza has more rubble than Ukraine, and to put that in perspective, the Ukrainian front line is 600 miles (nearly 1,000 kilometers) long, and Gaza is 25 miles (40 km) long," said Mungo Birch, head of the UNMAS program in the Palestinian territories.

But the sheer v...