A UN-appointed panel said today that Israeli forces violated international law, "including international humanitarian and human rights law", during and after their lethal attack on a flotilla of ships attempting to break the blockade of Gaza in May.

The UN Human Rights Council's fact-finding mission judged Israel's naval blockade of the Palestinian territory to be "unlawful" because there was a humanitarian crisis in Gaza at the time.

The panel's report, published today, described Israel's military response to the flotilla as "disproportionate" and said it "betrayed an unacceptable level of brutality".

Eight Turkish activists and one Turkish-American were killed in the raid, which prompted international criticism of both the attack and Israel's policy of blockading the Gaza Strip. Israel has since eased its embargo, although still refuses to allow full imports and exports and the free movement of people.

Israel says the soldiers acted in self-defence. But the mission criticised the Israeli government for failing to co-operate with its inquiry. "Regrettably to date, no information has been given to the mission by or on behalf of the government of Israel," it said.

The panel was led by Karl Hudson-Phillips, a retired judge of the international criminal court and former attorney general of Trinidad and Tobago.

The report said: "The conduct of the Israeli military and other personnel towards the flotilla passengers was not only disproportionate to the occasion but demonstrated levels of totally unnecessary and incredible violence. It betrayed an unacceptable level of brutality. Such conduct cannot be justified or condoned on security or any other grounds. It constituted grave violations of human rights law and international humanitarian law."

The panel concluded that there was "clear evidence" of wilful killing, torture or inhuman treatment and wilfully causing great suffering or serious injury to body or health – all crimes under the Geneva Convention.

The panel expressed the hope that there would be "swift action" by the Israeli government to help victims achieve effective remedies. "The mission sincerely hopes that no impediment will be put in the way of those who suffered loss as a result of the unlawful actions of the Israeli military to be compensated adequately and promptly," it said. It described the blockade of Gaza as "totally intolerable and unacceptable in the 21st century".

The Israeli government has fiercely resisted demands for an independent international inquiry into the flotilla attacks, establishing three internal investigations to avert pressure from the UN, Europe and Turkey.