UNITED NATIONS, Oct 18 (Reuters) - Cluster bombs dropped by Israel on southern Lebanon in its monthlong war with Hezbollah are still killing or injuring three to four civilians a day, a third of them children, land mine activists said on Wednesday.

Groups helping Lebanon clear unexploded mines from the war zone have identified 770 sites hit by cluster bombs during the conflict that ended in an Aug. 14 cease-fire, according to a new report by London-based Landmine Action.

While more than 45,000 unexploded cluster bomblets have been cleared and destroyed, hundreds of thousands more still litter the countryside and it will take another year or two to get the situation under control, the report said.

Cluster bombs burst into bomblets and spread out near the ground. While some aim to destroy tanks, others are designed to kill or maim humans over a wide area.

Experts have estimated an unusually high 40 percent of the bomblets dropped by Israel on Lebanon failed to explode on impact. A dud might resemble a soda can or a dusty rock and can be set off by as little as a touch, packing enough force to rip off a leg or kill a child.

Twenty deaths and 115 injuries have been reported in southern Lebanon from cluster munitions since the cease-fire, according to U.N. and Landmine Action figures as of Oct. 8. Children 18 and under accounted for four of the deaths and about 40 of the injuries.

Arms control activists argue that international law bans the use of such weapons in civilian areas.

Israel denies using the weapons illegally and has accused Hezbollah of firing rockets into Israeli territory from towns and villages, using their residents as human shields.

Thomas Nash, coordinator of the Cluster Munitions Coalition, said the unexploded bomblets were keeping children out of school, discouraging farmers from harvesting or planting their crops, blocking water supplies and slowing rebuilding.

Those farmers who dared to go into their fields despite the risk were paying the price, as 23 percent of the casualties since the cease-fire were tied to farming, he said.

"Israel's use of cluster munitions is just the latest example of the humanitarian harm that can be caused by these devices," Nash told U.N. delegates, activists and reporters.

While they have been in use for 40 years, there is no treaty explicitly governing their use, he said, adding the coalition had launched a campaign for one to be written.