GENEVA, Sept 22 (Reuters) - An attack by Israeli commandos on a Gaza-bound aid flotilla in May was unlawful and resulted in violations of human rights and international humanitarian law, a panel of international experts said on Wednesday.

The three experts, nominated by the United Nations Human Rights Council to investigate the Israeli attack in which nine pro-Palestinian activists -- eight Turks and one Turkish American -- were killed, also said that Israel's blockade of Gaza had caused a humanitarian crisis and was unlawful.

The experts -- judges from Britain and Trinidad and a Malaysian human rights campaigner -- said in a report that the Israeli military's action had used disproportionate force and totally unnecessary and incredible violence in intercepting the flotilla.

It betrayed an unacceptable level of brutality. Such conduct cannot be justified or condoned on security or any other grounds, they said in the report, to be submitted to the rights council on Sept. 27.

It constituted grave violations of human rights law and international humanitarian law.

The three experts said Israel had a right to security, and the firing of rockets into Israel from Hamas-controlled Gaza also constituted violations of humanitarian law.

But the Israeli blockade of Gaza amounted to collective punishment of the civilian population and was not lawful in any circumstances, they said.

The rights experts, who were not allowed to enter Israel, said Israel had refused to cooperate with their mission, and called on the Israeli authorities to identify those involved in the violence and prosecute them.

Israel, which says pro-Palestinian activists on the boat were killed when they attacked its commandos, had said from the outset it would not work with the probe by the rights council.

Many nations believe the council, on which Islamic states and their allies have a majority, focuses on Israeli treatment of Palestinians at the expense of other rights issues.

Israel has said it would cooperate with another UN probe convened by Secretary-General Ban Ki-moon into the incident, which damaged Israel's ties with Turkey.

Israel is also conducting its own inquiry.

Monday's session of the rights council will also examine another report by experts on the follow-up investigations by Israeli and Palestinian authorities into the 2008-2009 Gaza conflict. That report has found the probes were inadequate.