The UN had previously said it believed 350,000 unexploded cluster bomblets were in the south, but now estimates the figure at a million on the basis of Israeli media reports.
Chris Clark, the UN's top de-mining official in Lebanon, said: "The situation in south Lebanon now, as a result of 34 days of bombing, is that there is extensive unexploded ordnance lying all over the place."
Cluster bombs are anti-personnel weapons designed to break up above ground, scattering bomblets over a wide area.
Houses, gardens, farms and streets have been peppered with the munitions.
The UN also said that Israel's failure to hand over detailed information about the cluster bomb strikes has complicated and slowed the clearance of the unexploded bombs.
The UN says that it has repeatedly asked Israel to hand over information about the nature and location of the strikes with cluster munitions.
Fourteen people have been killed and about 90 injured from unexploded ordnance since the August 14 ceasefire, according to the United Nations Mine Action Centre in Lebanon.
Clark said it could take up to the end of 2007 to clear the munitions.
"The job is not over yet, the casualties will reach a record high this year"
Colonel Wassin Rizq,
head of operations,
Lebanese army's National Demining Office
Colonel Wassin Rizq, head of operations of the Lebanese army's National Demining Office, said: "The job is not over yet, the casualties will reach a record high this year."
David Shearer, the UN's humanitarian co-ordinator for Lebanon, said last week that Israel could assist the clearance effort by handing over co-ordinates of where the bombs were fired.
But Clark complained that Israel had not responded.
"We've not received this so far. We've asked and we are waiting," he said.
Rules of engagement
The UN assessment comes as Israel and international peacekeepers wrangle over the rules of engagement in south Lebanon that would enable Israeli troops to completely pull out of the area.
Israel has so far withdrawn from about 90 per cent of the land it occupied during the height of its 34-day war against Hezbollah, handing it to international peacekeeping forces under the expanded United Nations interim force in Lebanon (Unifil) mandate.
Amir Peretz, Israel's defence minister, said: "I hope very much that during the coming week, at the most a few more days, we will complete the withdrawal.
"There are arrangements, negotiations [and] deliberations which we intend to complete in order to set the rules - what is permitted and what is forbidden - from the moment we're sitting on the blue line [the international border]."