The UN says it estimates that 350,000 unexploded cluster bomblets remain in southern Lebanon, leaving a deadly legacy for civilians.
A leading Jewish group has also called on the UN's top human rights forum to include Hezbollah rocket attacks in its inquiry into the Lebanon conflict.
The Simon Weisenthal Center called for balance in the UN Human Rights Council probe.
The UN has so far identified 516 cluster bomb strike locations and says 30 to 40 percent of the bomblets they scattered over the south failed to explode at the time.
David Shearer, the United Nations humanitarian coordinator in Lebanon said: "The outrageous fact is that nearly all of these munitions were fired in the last three to four days of the war."
"Outrageous because by that stage the conflict had been largely resolved in the form of (U.N. Security Council) Resolution 1701," he said.
The resolution adopted on August 11 halted 34 days of fighting three days later. A truce has largely held since then.
Chris Clark, manager of the UN Mine Action Coordination Centre of South Lebanon, said the cluster bomb threat in the south was "extensive and, in my opinion, unprecedented".
While Israel has provided general information about where it believes unexploded ordnance might be, Clark said tactical maps given to the UN by Israeli forces withdrawing from the south were "absolutely useless" in clearance efforts.
Israel denies using cluster bombs illegally.
Representatives of the Simon Wiesenthal Center said after meeting the UN High Commissioner for Human Rights, Louise Arbour, that they had no objections to investigations in Lebanon, but called for balance in the probe.
Rabbi Abraham Cooper, the Center's associate dean said: "There is no doubt in our mind whatsoever that Hezbollah is guilty of war crimes."
"It is vital for a UN agency that uses the term human rights to play on a level playing field. We see this as a litmus test for the Human Rights Council," he told journalists.
The 47-member UN Human Rights Council last month voted to set up the probe into "systematic" Israeli attacks on civilians during the recent conflict in Lebanon.