WASHINGTON (AFP) - The United States pledged its unwavering commitment to Israel on Monday, and insisted it will take "whatever steps are necessary" to deny adversary Iran a nuclear bomb.

As international negotiators seek a deal with Teheran to rein in its nuclear program, Washington's UN ambassador Samantha Power reiterated President Barack Obama's commitment to prevent the Islamic republic from developing atomic weapons.

"He will keep his commitment whether the negotiations collapse or produce a diplomatic solution that meets our bottom line," she told some 16,000 attendees at US pro-Israel lobby AIPAC ahead of a speech by visiting Israeli Prime Minister Benjamin Netanyahu.

"Talk, no talk. Agreement, no agreement. The United States will take whatever steps are necessary to protect our national security and that of our closest allies," she assured.

"If diplomacy should fail, we know the stakes of a nuclear-armed Iran as well as everyone here. We will not let it happen," Power added.

She also stressed that the United States vowed to oppose the "ugly" attacks on the Jewish state's legitimacy through action in the United Nations.

"It is bitterly unjust that the United Nations, an institution founded upon the idea that all nations should be treated equally, is so often used cynically by member states to treat Israel unequally," Power said.

"These attacks on Israel's legitimacy are biased, they are ugly, and the United States of America will not rest until they stop," she said to sustained applause.

Power, also insisted the US-Israeli relationship - currently under strain over Iran's nuclear ambitions - "transcends politics, and it always will".