In a report that cited Palestinians who managed to get out of Khuza'a, HRW said the attacks on the town near the southern city of Khan Yunis occurred at the end of July.
"Israeli forces in the southern Gaza town of Khuza'a fired on and killed civilians in apparent violation of the laws of war in several incidents between July 23 and 25," said the New York-based watchdog.
"Deliberate attacks on civilians who are not participating in the fighting are war crimes."
The report pointed out that while Israeli forces had warned people to leave Khuza'a prior to July 21, "the failure of civilians to abide by warnings does not make them lawful targets of attack."
"Warning families to flee fighting doesn't make them fair targets just because they're unable to do so, and deliberately attacking them is a war crime," said HRW's Sarah Leah Whitson.