The Israeli army has launched an investigation into the shelling of a UN school in the Gaza Strip while dismissing several other allegations of misconduct during a bloody war last July-August.

The Palestinians are threatening Israel with action at the International Criminal Court (ICC) over alleged war crimes during the conflict, a move that could also open up the Palestinian Islamist movement Hamas which runs Gaza to investigation.

The Israeli military, for its part, is pressing dozens of internal probes into its conduct during the war, which killed nearly 2,200 Palestinians, mostly civilians, and 72 on the Israeli side, most of them soldiers.

"Factual findings... indicated the existence of grounds for a reasonable suspicion that the strike (on the school) was not carried out in accordance with the rules and procedures applicable to IDF (army) forces," the military attorney general said in a report released late Thursday.

"As a result, the MAG (military attorney general) has ordered the opening of a criminal investigation into the incident," it said.

On July 30, during the height of Israel's ground invasion of Gaza, shells hit a school run by the United Nations refugee agency for Palestinian refugees, UNRWA, that was being used as a shelter for the displaced, killing 17 people, according to the agency.

It was one of seven UNRWA schools hit during the war, during which a total of 83 schools were damaged by fire.

UNRWA spokesman Chris Gunness said he hoped a "proper investigation" would take place.

"On 28 separate occasions over the previous two weeks, we had informed the army of the coordinates" of the school in Jabalia, in northern Gaza, where nearly 3,000 people were sheltering, he told AFP.

"Right up to the night before (the strike), we were notifying them ... there are lots of unanswered questions" hanging over the incident.

"The figures cry out for proper investigation," he added.

The army in Thursday's report dismissed a number of other allegations after initial probes, including over the deaths of another 17 people the same day at a marketplace in northern Gaza.

It said the actions of the army in that and several other deadly incidents had "accorded with Israeli domestic law and international law requirements".

The marketplace shelling took place during a humanitarian ceasefire.

But the military said it was fired on first, and that the ceasefire did "not apply in a number of specific areas in which IDF forces were operating at that time".

Its response was "measured and limited in fashion."

The military also said it has launched criminal investigations into a number of cases of looting by Israeli soldiers.