President Barack Obama's nominee for UN ambassador on Wednesday slammed the global body's "unacceptable bias" against Israel, even as detractors criticize her past statements about America's closest Mideast ally.

Genocide expert and human rights champion Samantha Power enjoys bipartisan support in the US Senate, where broad cooperation this week on executive branch nominees was likely to ease her expected confirmation.

Power would replace Susan Rice, who leaves under a cloud for her role in the administration's public explanation for the deadly attack on the US mission in Libya last September 11 that four Americans dead, including the US envoy there.

Power, who also faulted the UN for failing to stop the slaughter in Syria, said in her testimony that America enjoys a "special relationship" with Israel, whose "legitimacy should be beyond dispute, and its security must be beyond doubt."

She added: "Within this (UN) organization built in the wake of the Holocaust... we also see unacceptable bias and attacks against the state of Israel."

Just 42 years old, Power is a foreign policy whiz who has won a Pulitzer Prize for a book on genocide, and was among those who nudged US policy towards ousting Libya's Moamer Kadhafi.

She has earned the backing of Republicans including veteran Senator John McCain, who has described her as capable and qualified.

But Retired Lieutenant General William 'Jerry' Boykin, who is behind a push to block Power's confirmation, reportedly blasted Power at an event early this month at the National Press Club.

"We should be proud to be Americans, and if you look at Samantha Power's track record there is a strong indication that her attitude is just the opposite," he said.

Boykin and other hawkish conservatives have pointed to controversial comments Power made in 2002 that some interpreted as being anti-Israel, in which she recommended the US stop spending on the Jewish state's military and instead invest billions of dollars in Palestinian entities.

Power has since disavowed those comments.

In recent months she has courted Jewish and pro-Israel groups, drawing support from the likes of the Anti-Defamation League.

In other remarks at her nomination hearing, Power spoke of "the absurdity of Iran chairing the UN Conference on Disarmament, despite the fact that its continued pursuit of nuclear weapons is a grave threat to international peace and security."

And on Syria, she pointed to "The failure of the UN Security Council to respond to the slaughter in Syria -- a disgrace that history will judge harshly."