Israel inflicted "massive and unprecedented harm" to Palestinian civilians in the 2014 Gaza war with indiscriminate fire and lax rules of engagement, a report said on Monday, citing testimony given anonymously by dozens of troops.
Israeli advocacy group Breaking The Silence said its accounts, gathered from dozens of unnamed soldiers, show a "change in the military's combat norms."
"A troubling picture arises of a policy of indiscriminate fire that led to the deaths of innocent civilians," director Yuli Novak said.
The 237-page report described how the IDF left swathes of devastation after they invaded Gaza last July with the stated aim of halting Hamas rocket fire out of the enclave.
"We were firing purposelessly all day long. Hamas was nowhere to be seen," one tank sergeant was quoted as saying.
The group said its finding cast "grave doubt on the IDF's ethics".The Israeli military says it tried to prevent non-combatant casualties, and accuses Hamas of inviting these by fighting from within residential areas. The Palestinian Islamist group has been criticized abroad for firing rockets at Israeli civilians.
Some 2,256 Palestinians were killed during the July-August conflict, of whom 1,563 were civilians, according to UN figures. On the Israeli side, 66 soldiers and six civilians (5 Israeli, 1 Thai) died. The lopsided tolls have fuelled foreign criticism of Israel and Palestinian calls for war-crimes suits.
Monday's report suggested the IDF looked to minimize the risk to their own troops even if that meant harming civilians.
An infantry sergeant is quoted as saying that, once Israel deemed civilians had mostly fled an area of operations, "there weren't really any rules of engagement".
"The idea was, if you spot something, shoot," he said. "If you shoot someone in Gaza it's cool, no big deal."
"Anything inside (the Gaza Strip) is a threat, the area has to be sterilized, empty of people and if we don't see someone waving a white flag, screaming, 'I give up,' or something then he's a threat and there's authorization to open fire," he added. Others described incidents in which civilians were killed.
Israel questioned the methodology and motivations behind the report. "Unfortunately, as in the past, Breaking the Silence has refused to provide the IDF with any proof of their claims," a military spokeswoman said.
"This pattern ... indicates that contrary to their claims this organization does not act with the intention of correcting any wrongdoings they allegedly uncovered."
Breaking the Silence, which listed the Swiss foreign ministry and the Norwegian embassy among its Western donors, said it collected testimony from more than 60 war veterans and called for an external probe "that can examine conduct at the highest ranks in the security and political establishments".
The group says its reports, which are critical of the military, are meant to show the Israeli public the difficult "reality" where soldiers serve in Palestinian areas.
The IDF has launched several internal investigations into the war and says these should suffice.
Publication of the report coincided with a conference organised by the Shurat Hadin Israel Law Center advocacy group, which argues Western democracies pitted against militants like Hamas require a "new law of war".
Keynote speaker Benny Gantz, Israel's armed forces chief during the Gaza war, defended his troops' conduct as legal and predicted bloodier conflict in the future given the difficulty in distinguishing between Palestinian fighters and civilians.
"Next time, it will be worse, because Israel has to constantly grapple with the moral dilemma, but we need to protect our country," Gantz said. Asked by Reuters about the Breaking the Silence report, the ex-general declined comment.
The Associated Press contributed to this report.