GENEVA (AFP) - Hollywood star George Clooney's fiancee Amal Alamuddin has declined her nomination to join a commission probing Israel's Gaza offensive, the head of the UN Human Rights Council said on Tuesday, while Israel is said to be furious over the appointment of Canadian international lawyer William Schabas to head the commission.

Mr Schabas is widely regarded in Israel as being hostile to the Jewish state over reported calls to bring Prime Minister Benjamin Netanyahu before the International Criminal Court.

In a statement, UN Human Rights Council head Baudelaire Ndong Ella said that Lebanese-born British lawyer Ms Alamuddin had cited "prior professional commitments and regrets that the commission will not benefit from her expertise in the field".

Mr Ella, who this year holds the rotating presidency of the top United Nations rights forum, had on Monday named Ms Alamuddin to the three-member commission of inquiry.

He said that he had approached a "number of individuals" as potential candidates before making the announcement, and that Ms Alamuddin had several hours later said that she was not in a position to accept the role.

Despite Ms Alamuddin's decision, the commission of inquiry is now operational, Mr Ella said, adding that he would "decide on the way forward".

The commission will be led by Canadian international lawyer William Schabas, and also include Doudou Diene of Senegal, who has previously served as the UN's watchdog on racism and on post-conflict Ivory Coast.

The UN Human Rights Council ordered the Gaza investigation on July 23, in the face of fierce opposition from Israel and the United States.

The decision came during a marathon seven-hour emergency session of the 47-nation council, where Israeli and Palestinian delegates traded accusations over each sides' alleged war crimes.

The probe team has been tasked with reporting back to the council by March.

Ms Alamuddin's family, who are from Lebanon's Druze community, fled to Britain during the country's 1975-1990 civil war.

The 36-year-old, who is fluent in Arabic, French and English, is reportedly due to wed 53-year-old Clooney in Italy in September.

News that she had stolen the heart of one of Hollywood's most celebrated bachelors caused a global media frenzy back in April.

Ms Alamuddin is well-versed in international conflict probes. She worked with the international tribunal examining the assassination of former Lebanese prime minister Rafiq Hariri, and assisted former UN head Kofi Annan in efforts to make peace in Syria.

Among her legal clients have been Ukraine's former prime minister Yulia Tymoshenko and controversial Wikileaks founder Julian Assange.

Israel lashed out on Tuesday over the appointment of Mr Schabas.

"This commission's anti-Israeli conclusions have already been written, all it needs is a signature," railed foreign ministry spokesman Yigal Palmor.

"For this commission the important thing is not human rights but the rights of terrorist organisations like Hamas," he told AFP.

But in a series of interviews with the Israeli media, Mr Schabas defended himself against allegations of bias against the Jewish state.

"I've frequently lectured in Israel, at universities in Israel, I'm a member of the editorial board of the Israel law review, I wouldn't do those things if I was anti-Israel," he told public radio.

He challenged Mr Palmor's assertion that the commission's findings were a foregone conclusion.

"As far as I'm concerned they're not written at all, that's the whole point of an investigation," he told the radio.

"Many of the questions we have to examine will deal with very precise matters on which the generalities about the conflict don't provide any insight.

"When we look at specific incidents in which... civilians were killed during the conflict, there are issues about targeting, about proportionality, each one of these has to be examined specifically."

In a second interview with Israel's army radio, he said that he would also be looking into the actions of Palestinian militants.

"The mandate that the commission has been given doesn't specify this and I think a reasonable interpretation would be that mandate requires you to look at both sides," he said.

He said the commission's findings are to be published in March 2015.

Israel has long had stormy relations with the UNHRC.

In January 2012, it became the first country to refuse to attend a periodic review of its human rights record.

And two months later, it cut all ties with the Geneva-based council after it announced an inquiry into how West Bank settlements may be infringing on Palestinians rights.

Israel has accused the UNHRC of routinely singling it out at its annual meetings, as well as passing a number of anti-Israel resolutions.