Carolyn Kaster/AP Photo

President Obama warned Iran before the U.N. General Assembly today that the U.S. will "do what we must" to prevent it from obtaining a nuclear weapon.

"Time and again, it has failed to take the opportunity to demonstrate that its nuclear program is peaceful, and to meet its obligations to the United Nations," he said.

Amid rising tension over Iran's nuclear ambitions, the president argued there is still "time and space" for diplomacy, but made clear "time is not unlimited."

"We respect the right of nations to access peaceful nuclear power, but one of the purposes of the United Nations is to see that we harness that power for peace. Make no mistake: a nuclear-armed Iran is not a challenge that can be contained," the president said.

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Obama, however, stopped short of giving Iran an ultimatum, or drawing a "red line" it cannot cross, as Israeli Prime Minister Benjamin Netanyahu has demanded.

A nuclear Iran, Obama said, "would threaten the elimination of Israel, the security of Gulf nations, and the stability of the global economy. It risks triggering a nuclear-arms race in the region, and the unraveling of the non-proliferation treaty."

"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 said.