Vice President Joe Biden says President Barack Obama isn't bluffing when he says he'll use military action if ultimately necessary to prevent Iran from acquiring a nuclear weapon.

American Israel Public Affiars Committee President Michael Kassen (L) welcomes Vice President Joe Biden to the podium during AIPAC's annual policy conference at the Washington Convention Center (Chip Somodevilla/Getty Images)

He told a powerful pro-Israel lobby's annual conference on Monday that protecting Israel is in the United States' interest. On Iran, Biden says the U.S. still prefers a diplomatic option but that the window for that is closing.

But Biden is cautioning against acting too hastily. He says every other option must be exhausted to ensure the world community will be supportive if there's a need for a military intervention.

Biden says efforts to delegitimize Israel as a Jewish state are the most dangerous change he's seen as it related to Israel's security. He says Israel's legitimacy is non-negotiable for the U.S.

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