A U.N. report last month only reinforced Israeli fears, finding that Iran has moved more of its uranium enrichment activities into fortified bunkers deep underground where they are impervious to air attack. Enrichment is a key activity in building a bomb, though it has other uses as well, such as producing medical isotopes.
While Israel is convinced that Iran is pursuing a nuclear weapon, American officials believe Iran has not yet made a final decision to take the plunge, even as it develops much of the infrastructure needed to do so.
Obama has repeatedly said he will not allow Iran to gain nuclear weapons and has said the U.S. would be prepared to use force as a last resort. But in an interview Sunday with "60 Minutes" he also vowed to "block out any noise that's out there" on the issue, in an apparent swipe at Netanyahu.
Israel's timeline for military action is shorter than that of the United States, which has far more powerful bunker-busting bombs at its disposal, and there is great suspicion in Israel over whether in the moment of truth Obama will follow through on his pledge.
Netanyahu has a history of fiery speeches at the U.N. General Assembly.
In 2009, he waved the blueprints for the Nazi death camp Aushwitz and invoked the memory of his own family members murdered by the Nazis while making his case against Iran's Holocaust denial and threats to destroy Israel.
To those who remained at the General Assembly while Iranian President Mahmoud Ahmadinejad spoke, he chastised: "Have you no shame? Have you no decency?"
And last year, he warned the world about the threat of militant Islam and Iran.
During his three-day visit, Netanyahu is scheduled to meet with U.N. Secretary-General Ban Ki-Moon, EU foreign policy chief Catherine Ashton, Canadian Prime Minister Stephen Harper — who recently expelled the Iranian ambassador from his country — and U.S. Secretary of State Hillary Clinton. He is not scheduled to meet Obama.
Palestinian President Mahmoud Abbas is also set to speak in New York. The Palestinian leader is expected to ask the United Nation's General Assembly to vote on recognizing Palestine as an observer state in November.
Abbas has said he wants to put off a vote until after U.S. elections to avoid entangling the Palestinian statehood bid in presidential politics.
"I'm not in the U.N. to delegitimize Israel but to present the two state solution," Abbas told U.S. Jewish community leaders last week at a meeting by the S. Daniel Abraham Center for Middle East Peace. "I want to have Palestine as a nonmember state." said.
Abbas' relatively low-key approach is in contrast to last year when he was at center stage with his attempt to win full membership in the world body. That application failed to win enough support in the Security Council.