Israeli forces "did not allow the evacuation of injured civilians who were besieged for days at a time and left the civilians without food or water for considerable periods", the group said.
PHR said its findings indicate a "dangerous and retrograde trend" of an increasing disregard for the obligation to protect medical personnel during Israeli operations.
Israel's military said the group's allegations are still being investigated.
"The findings ... have not yet been concluded. When they have been finalized, (they) will be presented to the public," the military said in a statement.
But the military said its forces had been instructed to "act with the utmost caution in order not to cause harm to medical vehicles and medical facilities".
It said Hamas fighters used medical vehicles, facilities and uniforms to "conceal and camouflage terrorist activity" and made use of ambulances to carry weapons and fighters.
Last week, the United Nations special rapporteur on human rights in the Palestinian territories said Israel's offensive on Gaza "would seem to constitute a war crime of the greatest magnitude under international law".
Richard Falk called the bombardment "an inhumane form of warfare" in a report to be submitted to the UN Human Rights Council.
Israel's government has rejected Falk's assertions.