Amnesty International on Thursday accused Israel and Hamas of having committed “serious and extensive” violations during Israel’s assault in the Gaza Strip and urged the international community to boost its support for a United Nations inquiry into the alleged war crimes.

In its 119-page report, the London-based human rights group also called on Israel and Hamas, the Islamist group that controls Gaza, to fully co-operate with the UN probe headed by Richard Goldstone, an ex-chief prosecutor at the international criminal tribunals for the former Yugoslavia and Rwanda.

Mr Goldstone’s team, which is aiming to publish its report in September, held public hearings in Gaza City on Sunday and Monday to broadcast live witness accounts from Palestinians as part of the investigation. It plans another round of hearings in Geneva next week, for which some Israeli witnesses may be flown in.

Israel has refused to co-operate with the probe, claiming it was biased against the Jewish state, and has deprived the team access to military sources and victims of Hamas rockets. While Hamas has said it would help the mission, the fact that its officials have often accompanied the investigators in Gaza has drawn scepticism about the ability of witnesses to freely describe the militant group’s actions.

Gaza health officials and local rights activists have said that some 1,400 Palestinians were killed during the 22-day attacks launched by Israel in December in the country’s bid to curtail rocket fire on its southern communities by Gaza militants. Thirteen Israelis were also killed during the onslaught.

Israel has rejected charges that it had committed war crimes and has cast doubt on such allegations by claiming that they relied on questionable sources that sided with Hamas. On Tuesday, Human Rights Watch accused Israel in a separate report of unlawfully killing at least 29 Palestinian civilians, including eight children, with missile-firing drones during the military operation.

In its report, Amnesty criticised Israel for failing to launch an independent probe into the conduct of its forces and pressed the country to “provide full reparations for the consequences of its unlawful acts and omissions.” The group also insisted that its own delegates were allowed to carry out their inquiries in Gaza unhindered by Hamas officials and often heard criticism of Hamas’s conduct from Gazans.

According to Amnesty, hundreds of Palestinians had been killed during the conflict as a result of Israeli forces being reckless or deliberately targeting civilians by shooting at close range at those fleeing their homes in search of shelter, firing tank shells into civilian homes or dropping aircraft bombs on residential buildings. The group added that in all the cases that it investigated, it found no evidence that the victims posed a threat to the Israeli soldiers or were caught in an area of combat between the soldiers and Palestinian militants.

Amnesty said it also discovered no evidence of Israeli allegations that Hamas or any other Palestinian factions had used civilians as human shields, including by forcing them to stay in or around buildings used by gunmen. The group, however, said it found that Israeli soldiers themselves used civilian homes as bases and sniper positions while forcing families to stay in a ground-floor room, or ordered males to walk in front of them on streets or check for booby traps or militants inside buildings.

Amnesty’s report added that Israel’s blockade on the impoverished territory of 1.5m Palestinians is hampering any reconstruction efforts following the attacks, which destroyed or damaged many houses, businesses, public buildings and farms.