US-based Human Rights Watch says Israel has used cluster bombs in civilian areas during its assault on Lebanon.
Israel says its use of cluster bombs is legal under international law
The group says an attack using the munitions on the village of Blida last week killed one person and injured 12.
It says the explosives - which disperse after impact - are "unacceptably inaccurate and unreliable", and should not be used in populated areas.
The Israeli military says their use is legal under international law, and that it is investigating the Blida incident.
Critics say cluster bombs leave behind a large number of unexploded bomblets, which often kill long after they are fired.
"Our research in Iraq and Kosovo shows that cluster munitions cannot be used in populated areas without huge loss of civilian life," said Kenneth Roth, executive director of Human Rights Watch (HRW).
The group believes that the use of cluster munitions in populated areas may violate the prohibition on indiscriminate attacks contained in international humanitarian law.
"They're not illegal per se, but certain attacks may be illegal," Washington representative Bonnie Docherty says.
"The law of war requires you to distinguish between soldiers and civilians, so when you are using an outdated, unreliable weapon in a populated area, it is likely that the attack violates international humanitarian law," she told the BBC.
"We have researchers on the ground who are investigating them and will investigate other claims related to cluster munitions, as well as other incidents in the ongoing conflict."
Phosphorous bombs claims
Separately, there have been reports in Lebanon that Israel is using phosphorous bombs in its offensive.
Doctors in hospitals in southern Lebanon say they suspect some of the burns they are seeing are being caused by phosphorous bombs.
Jawad Najem, a surgeon at the hospital in Tyre, told the Associated Press news agency that patients admitted on Sunday were burn cases that resulted from Israeli phosphorous incendiary weapons.
Lebanese President Emile Lahoud made an oblique reference to their use in an interview with French radio.
"According to the Geneva Convention, when they use phosphorous bombs and laser bombs, is that allowed against civilians and children?" he said on Monday.
The Geneva Conventions ban the use of white phosphorous as an incendiary weapon against civilian populations and in air attacks against military forces in civilian areas.
An Israeli military spokeswoman said arms used in Lebanon did not contravene international norms.
"Everything the Israeli Defence Forces are using is legitimate," the spokeswoman was quoted as saying by Reuters news agency.
The Israeli military says it is investigating the claims.