Up to a million cluster bomblets discharged by Israel in its conflict with Hezbollah remain unexploded in southern Lebanon, the UN has said.
There are three times more live devices than previously thought
The UN's mine disposal agency says about 40% of the cluster bombs fired or dropped by Israel failed to detonate - three times the UN's previous estimate.
It says the problem could delay the return home of about 200,000 displaced people by up to two years.
The devices have killed 14 people in south Lebanon since the August truce.
The manager of the UN's mine removal centre in south Lebanon, Chris Clark, said Israel had failed to provide useful information of its cluster bomb strikes, which could help with the clearance operation.
Last month, the UN's humanitarian chief, Jan Egeland, accused Israel of "completely immoral" use of cluster bombs in the conflict.
Israel says all its weapons and munitions, as well as their use, comply with international law.
'Threat to life'
Mr Clark said Israel fired up to 6,000 bombs, rockets and artillery a day into Lebanon during the 34-day conflict.
He said more than 40,000 cluster bomblets had been cleared since the fighting ended on 14 August, but many more remained scattered "in bushes, trees, hedges and wire fences".
Canisters packed with hundreds of bomblets
Fired from the ground or dropped by aircraft
Bomblets dispersed while bombs in mid-air
Mr Clark said information Israel had provided to help with the bomblets' clearance had been "useless".
"We have asked for grid references for [cluster bomb] strikes," he said.
"We have not received them so far."
The UN's refugee agency said the danger of unexploded cluster bombs meant some 200,000 people displaced by the conflict would not be able to return home for up to two years, rather than 12 months as previously forecast.
"This is clearly the biggest threat to civilian life," said Arjun Jain, of the United Nations High Commissioner for Refugees.
Hundreds of bomblets are packed into the cluster bombs, which are fired from the ground or dropped by aircraft.
The bombs detonate in mid-air, dispersing the drinks-can sized bomblets over a wide area. Those which do not explode on impact become like anti-personnel mines.
The use of cluster bombs is not prohibited under international law.