Mr Egeland told a news briefing Israel had either made a "terribly wrong decision" or had "started thinking afterwards".
The remarks were seen as unusually harsh, even for Mr Egeland, who often violates an unwritten rule that UN officials should not criticise member states too severely.
"What's shocking and I would say, to me completely immoral, is that 90 per cent of the cluster bomb strikes occurred in the last 72 hours of the conflict, when we knew there would be a resolution," Mr Egeland said at a news conference.
The cluster bombs used in the war had an unusually high failure rate, possibly because they were old, Mr Egeland said.
Usually 10 to 15 per cent of the bomblets fail to explode immediately, but research has estimated that up to 70 per cent of the Israeli bomblets failed to explode initially.
Unexploded munitions
Civilians returning to their homes in southern Lebanon are experiencing "massive problems" as a result of these unexploded munitions, Mr Egeland said.
Approximately 250,000 Lebanese, of the one million displaced, cannot move back into their homes because they have either been destroyed or are littered with unexploded munitions.
"Every day people are maimed, wounded and are killed by these ordnance," Mr Egeland said.
He inferred that this loss of life - and the late-stage bombing that is responsible for it - "shouldn't have happened".
Thirteen people, including three children, have been killed between the August 14 ceasefire and yesterday, and researchers with Human Rights Watch said the density with which Israel used cluster bombs in southern Lebanon was worse than any place they had seen.
Mr Egeland urged those countries which had manufactured the cluster bombs and sold them to the Israelis, including the United States, to have "serious talks with Israel".
The UN Mine Action Co-ordination Centre, which has so far assessed 85 per cent of the bombed areas in Lebanon, has identified 379 bomb strike areas that are contaminated with as many as 100,000 unexploded bomblets.
Lebanon appeal
Nonetheless, Mr Egeland called the current situation in Lebanon "remarkable" because about 750,000 people had managed to return home in the span of only a few days.
Mr Egeland will travel to Stockholm tomorrow to launch a revised humanitarian appeal for Lebanon.
The appeal has already essentially met its goal of A$118 million, which Mr Egeland indicated would satisfy the needs of the first emergency response phase.
The Lebanese government will launch its own appeal for several hundred million dollars in order to continue the rebuilding.
The meeting in Stockholm will also address the humanitarian crisis in Gaza, which Mr Egeland labelled a "ticking time bomb".