Israel lashed out on Tuesday after the UN Human Rights Council named the man who will be running an inquiry into its Gaza offensive.
Canadian international lawyer William Schabas, who will head the commission, is widely regarded in Israel as being hostile to the Israeli state over reported calls to bring Prime Minister Benjamin Netanyahu before the International Criminal Court.
"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, Schabas defended himself against allegations of bias against Israel.
"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 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.