A group of veteran Israeli combatants has 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 on Monday 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," the report 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.

The military raised doubts over 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," it said.

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