President Barack Obama pledged in a speech to world leaders Tuesday that the United States will do what it takes to prevent Iran from building a nuclear weapon and warned that time for a diplomatic resolution “is not unlimited.”

While there is still a chance to negotiate, Obama told the annual meeting of the United Nations General Assembly in New York, a nuclear-armed Iran would imperil Israel, ignite a regional arms race and destabilize the global economy.

“Make no mistake: A nuclear-armed Iran is not a challenge that can be contained,” Obama said in a speech aimed at two distinct audiences: Mideast leaders, including those of new governments emerging from the Arab Spring, and U.S. voters who in six weeks choose between him and Republican Mitt Romney in the presidential election. “The United States will do what we must to prevent Iran from obtaining a nuclear weapon.”

The Middle East and North Africa were the focus for Obama in his U.N. speech, and he used the bulk of the address to talk about the attacks on U.S. diplomatic outposts triggered by an anti-Islam video made in the U.S. The deadliest was a Sept. 11 assault on the U.S. consulate in Benghazi, Libya, that killed Ambassador Chris Stevens and three other Americans.

The violence has opened a line of attack by Romney on Obama’s foreign policy, particularly his handling of relations with Israel and the threat posed by Iran’s nuclear ambitions. In an address to the Clinton Global Initiative in New York just before Obama spoke, Romney said the U.S. seems “at the mercy of events rather than shaping events.”

At the U.N., Obama called the assaults in Libya “attacks on America” and vowed the U.S. would be “relentless” in tracking down the killers.

While calling the video “crude and disgusting” and saying the U.S. government had nothing to do with it, Obama said such expression can’t be banned in a free society.

“Americans have fought and died around the globe to protect the right of all people to express their views—even views that we disagree with,” Obama said. “We do so not because we support hateful speech, but because our founders understood that without such protections, the capacity of each individual to express their own views, and practice their own faith, may be threatened.”

In an otherwise somber speech, Obama drew some laughter from his audience with a self-reflective defense of free speech. “Here, in the United States, countless publications provoke offense,” he said, and as president “I accept that people are going to call me awful things every day, and I will always defend their right to do so.”

He warned that newly elected leaders in countries that were swept by the Arab Spring uprising are threatened by the same anger and extremism now aimed at the U.S.

“The impulse towards intolerance and violence may initially be focused on the West, but over time it cannot be contained,” Obama said. “The same impulses toward extremism are used to justify war between Sunnis and Shia, between tribes and clans.”

While saying the future must not belong to those who slander Islam, Obama said, “Those who condemn that slander must also condemn the hate we see when the image of Jesus Christ is desecrated, churches are destroyed, or the Holocaust is denied.”

Obama restated his call that the regime of Syria’s Bashar Assad “must come to an end.”