A group of veteran Israeli combatants on Monday slammed the army's conduct during the 2014 Gaza war, using soldiers' testimonies to expose a policy it said caused "unprecedented" civilian casualties.

The testimonies appeared in a 237-page report called "This is how we fought in Gaza" which was based on the anonymous accounts of 60 soldiers and officers involved in the 50-day war last summer.

They were collated by Breaking the Silence, an Israeli non-governmental organisation whose members bear witness to abuses they have seen or taken part in during their military service in the occupied Palestinian territories.

But the Israeli army questioned the veracity of the report, saying the group had failed to provide "proof" of the witness accounts.

"The guiding military principle of 'minimum risk to our forces, even at the cost of harming innocent civilians' alongside efforts to deter and intimidate the Palestinians, led to massive and unprecedented harm to the population and the civilian infrastructure in the Gaza Strip," it said.

The report gave more than 100 examples of apparent misconduct during the war.

One soldier related how troops were instructed to "open fire everywhere, first thing as you go in... the assumption being that the moment we went in, anyone who dared poke his head out was a terrorist".

"The instructions were to shoot right away. Whoever you spot -- be they armed or unarmed, no matter what. The instructions are very clear. Any person you run into, that you see with your eyes -- shoot to kill," said another.

"It's an explicit instruction."

A separate testimony said that two women who were talking on phones just under a kilometre away from the front line were shot dead.

When the bodies were later discovered to be unarmed, "we moved on, and they were listed as terrorists", the soldier said.

- 'Investigation impossible' -

Tanks were also ordered to fire at buildings, selected at random, in memorial of fallen comrades, the report cited a soldier as saying.

"When you look at the bigger picture, that's something we were doing all the time. We were firing purposelessly all day long," the soldier said.

The report also said junior officers were allowed to give orders that would normally be left up to more senior commanders.

"A very disconcerting picture arises about the way IDF forces were instructed to operate during combat in Gaza," it said, referring to the Israel Defence Forces, or army.

The military said it had doubts about the veracity of the report.

"The IDF is committed to properly investigating all credible claims raised via media (and) NGOs," it said in a statement.

"Unfortunately, as in the past, Breaking the Silence has refused to provide the IDF with any proof of their claims. For obvious reasons such conduct makes any investigation by the relevant IDF bodies impossible," it said.

But Breaking the Silence co-founder Yehuda Shaul told AFP the organisation had sent a letter to Israel's army chief of staff on March 23 asking for a meeting.

"We were very clear that we would be more than happy to share materials once we have a meeting scheduled," he said.

"Sadly enough, we never heard back."

The fighting claimed the lives of about 2,200 Palestinians, mostly civilians, and 73 people on the Israeli side, mostly soldiers.

The Israeli army is currently investigating allegations of misconduct, including the shelling of a United Nations-run school.

Last week, a UN report found Israel responsible for seven incidents at such schools which killed dozens of Palestinians sheltering there.

As part of its investigations so far, the Israeli military has charged three soldiers with looting during the war.