A senior researcher at Human Rights Watch, who has studied both the Kosovo and Lebanon conflicts, Fred Abrahams, said he was concerned that Israel was not paying enough attention to international legal requirements for "distinction and proportionality - first, to distinguish between combatants and civilians, and second, whether an attack will have a disproportionate effect on the civilians in the area."
Even if a target is legitimate, he said, "you can't drop a 500-pound bomb in an area crowded with civilians."
This was also the first conflict he could remember when civilians could not flee the war zone. Gaza's borders are shut both to Israel and to Egypt.
"Civilians are fish in a barrel," he said according to the Islamic republic news agency.
"Our conclusions are preliminary, but evidence is suggesting serious violations of the laws of war, which require investigation," Abrahams said.
That is also true of Hamas, he said. "We need to know more about what Hamas is doing on the ground," he said.
Regarding force protection, he said it "must be balanced by distinction and proportion."
"A violation by Hamas shooting from a mosque or school doesn't give the Israeli army carte blanche to return fire in the name of force protection with everything and anything it has," he added.
Human rights groups are also concerned about the Israeli use of white phosphorous, which creates smoke on a battlefield, at low altitudes or crowded areas, because it can burn like a kind of napalm.