Samantha Power, President Obama's nominee to be the US ambassador to the United Nations, has promised to combat what she called the “unacceptable bias” against Israel at the global body.
The decision to appoint Mrs Power raised initial concerns in Jerusalem due to her past statements on the Israeli-Palestinian conflict.
She has previously recommended that the US divest its support from Israel's military and devote billions to "a mammoth protection force”.
But during her US Senate confirmation hearing on Wednesday, the 42-year-old human rights advocate and former journalist said: “The United States has no greater friend in the world than the State of Israel.
“Just as I have done as President Obama's UN adviser at the White House, I will stand up for Israel and work tirelessly to defend it.”
"Israel—not Iran, not Sudan, not North Korea—is the one country with a fixed place on the Human Rights Council's agenda. Israel's legitimacy should be beyond dispute, and its security must be beyond doubt," Mrs Power said.
She also pledged to lobby for Israel to gain a seat on the UN Security Council, a US policy that has been blocked by many other countries.
"The Security Council seat is one that has eluded Israel, despite its many contributions across the years, and I commit to you wholeheartedly to go on offense, as well as playing defense on the legitimation of Israel, and we'll make every effort to secure greater integration of Israeli public servants in the UN system."
During her hearing Mrs Power also addressed the civil war in Syria, saying: "We see the failure of the UN Security Council to respond to the slaughter in Syria - a disgrace that history will judge harshly."
She added that President Bashar al-Assad's government had "written a new playbook for brutality”. She also advocated a tougher stance to halt Iran’s nuclear programme.
Senator Bob Corker, the top Republican on the committee, voiced his support for Mrs Power, calling her “a significant and positive force at the United Nations”.