President Barack Obama vowed Tuesday the United States would not permit Iran to arm itself with nuclear weapons, as the UN General Assembly heard appeals for an end to Syria's civil war.
Crises across the Middle East dominated the first day of the annual UN summit, where Western leaders sought to increase pressure on Damascus and Iran to abandon confrontation and seek negotiated settlements.
UN Secretary General Ban Ki-moon condemned the violence, speaking a day after his peace envoy had accused Syrian leader Bashar al-Assad's regime of resorting to "medieval forms of torture" against his own citizens.
And Obama went further, declaring: "the regime of Bashar al-Assad must come to an end so the suffering of the Syrian people can stop, and a new dawn can begin."
Obama was unequivocal on Iran, which is locked in a stand-off with the West over a nuclear program that Washington alleges is designed to produce a weapon that could tip the balance of power in an already volatile region.
"Make no mistake. A nuclear-armed Iran is not a challenge that can be contained. It would threaten the elimination of Israel, the security of Gulf nations, and the stability of the global economy," Obama said.
"That is why a coalition of countries is holding the Iranian government accountable. And that is why the United States will do what we must to prevent Iran from obtaining a nuclear weapon," he declared.
Six weeks before he is due to seek re-election, Obama is under pressure on the foreign policy front, with criticism of his handling of the killing of US diplomats and claims he is not standing closely enough behind Israel.
His speech was designed to counter claims from White House rival Mitt Romney and also to renew his outreach to the Muslim world after two weeks of anti-American violence triggered by a movie trailer that insulted Islam.
Obama said the Arab Spring would lead to improved democracy and living standards in a Middle East region more in line with US values but, while he condemned the film, he insisted no insults could justify violence.
He vowed that the militants who stormed the US consulate in Benghazi on September 11, killing the American ambassador to Libya and three colleagues, would face justice and said the United States would always defend free speech.
"There are no words that excuse the killing of innocents. There is no video that justifies an attack on an embassy," he said.
"There is no slander that provides an excuse for people to burn a restaurant in Lebanon, or destroy a school in Tunis, or cause death and destruction in Pakistan," he continued.
Debate in New York in the run-up to the assembly focused on the violence in Syria and the risk that the Iranian stand-off could lead to broader conflict if Israel or the United States launched a pre-emptive strike.
Ban spoke for many delegates when he called on world powers to put aside their differences and unite behind a plan to pressure the parties to ensure the conflicts be settled through negotiation.
Ban dubbed the Syria conflict "a regional calamity with global ramifications" and said: "The international community should not look the other way as violence spirals out of control."
"We must stop the violence and flow of arms to both sides and set in motion a Syrian-led transition as soon as possible," Ban added.
The 15-nation Security Council has become paralyzed by deadlock over the 18-month-old war, which Syrian activists say has left more than 29,000 dead.
Ban also expressed concerns for the mounting tensions surrounding Iran, denouncing what he called: "The shrill war talk of recent weeks."
While Obama took no option off the table in dealing with Iran's nuclear ambitions, Ban called on the United Nations to reject threats of military action, warning: "Any such attacks would be devastating."
Iran's President Mahmoud Ahmadinejad, who is in New York, has shrugged off talk of an attack on his country's nuclear facilities, and insisted that his country will not end he insists is peaceful civilian research.