“The United States will do what we must to prevent Iran from obtaining a nuclear weapon,” Obama told UN delegates.

In his speech, Obama also delivered a spirited call for reconciliation and tolerance in the wake of the deadly anti-American violence that has surged through the Muslim world since a video defaming the Prophet Mohammed was posted to YouTube earlier this month.

Obama’s comments on Iran come amid heightened pressure from Israeli Prime Minister Benjamin Netanyahu in recent weeks. Netanyahu has called on the United States to draw a “red line” to halt Iran’s nuclear ambitions.

The U.S. president’s warning also came just a day after Iranian President Mahmoud Ahmadinejad made incendiary statements concerning Israel, saying it has “no roots” in the Middle East. Ahmadinejad is set to address the UN General Assembly on Wednesday.

Iran has insisted its nuclear program is strictly for peaceful purposes.

Obama has maintained that diplomacy and sanctions should be the main levers to prevent Iran from developing a nuclear weapon, though he has not ruled out military action.

"Make no mistake a nuclear armed Iran is not a challenge that can be contained," Obama told the General Assembly, adding that there is “still time and space” to resolve the issue diplomatically.

“But that time is not unlimited,” he said.

Obama also accused Tehran of supporting terrorist groups outside the country, and he pivoted to the ongoing civil war in Syria by accusing Iran of propping up Syrian strongman Bashar al-Assad in the increasingly bloody conflict.

The United States and its allies have repeatedly accused Assad of war crimes against his own people and insisted he give up power. UN Security Council members Russia and China, however, have refused to join the international chorus calling for Assad’s ouster.

“We again declare that the regime of Bashar al-Assad must come to an end so that the suffering of the Syrian people can stop, and a new dawn can begin,” Obama told UN delegates.

Russia has warned that taking sides in the type of civil war ravaging Syria can lead to dangerous unintended consequences.

In his speech Tuesday, Obama also defended America’s support of the so-called “Arab Spring” revolutions that swept across the Middle East and North Africa last year. The uprising has left a turbulent political and security landscape in Egypt, Libya, Tunisia and elsewhere, where there have been violent anti-American protests.

Obama paid tribute to J. Christopher Stevens, the U.S. ambassador to Libya who was killed by armed militants during an attack on a U.S. diplomatic compound in Benghazi earlier this month. The deadly assault came amid widespread outrage in the Muslim world sparked by the YouTube video “The Innocence of Muslims,” produced by a Coptic Christian in California.

Obama called the film “crude and disgusting” but said no speech, however insulting, should be used to justify violence.

“There are no words that excuse the killing of innocents,” Obama said. “There is no video that justifies an attack on an embassy.”

The attacks on U.S. embassies represent an “assault on the very ideals upon which the United Nations was founded,” Obama added.

"If we are serious about these ideals, we must speak honestly about the deeper causes of this crisis,” he said. “Because we face a choice between the forces that would drive us apart, and the hopes that we hold in common."

Republican presidential challenger Mitt Romney on Tuesday addressed the Obama administration’s recent difficulties in dealing with the anti-American violence in the Muslim world.

"We somehow feel that we are at the mercy of events, rather than shaping events," Romney said in a speech at the annual meeting of the Clinton Global Initiative in New York.

Romney was introduced at the event by former President Bill Clinton, who has been stumping for Obama in the run-up to the November 6 presidential election.