Obama warns Iran, denounces Libya attack at UN

President Barack Obama will warn Iran on Tuesday that America will "do what we must" to prevent Tehran from acquiring a nuclear weapon, and will scold violent demonstrators who have targeted American diplomatic posts across the Muslim world.

"The attacks of the last two weeks are not simply an assault on America. They are also an assault on the very ideals upon which the United Nations was founded," Obama said in excerpts of his speech to the annual United Nations General Assembly, released by the White House.

"There are no words that excuse the killing of innocents. There is no video that justifies an attack on an Embassy. 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 said.

The remarks, to be delivered on one of the largest stages of international affairs, will carry a political message, too. Mitt Romney has accused Obama of being too weak on Iran, breaking faith with Israel, and botching the response to the so-called "Arab Spring" uprisings that swept authoritarian rulers from power in the Middle East and North Africa. The Republican standard-bearer also accused Obama of siding with those who stormed the U.S. Consulate in Benghazi, Libya, in an attack that left four Americans dead, including Ambassador Chris Stevens.

In his speech, Obama will warn Iran that "time is not unlimited" for resolving the tense standoff over its nuclear program through peaceful means.

"Make no mistake: a nuclear-armed Iran is not a challenge that can be contained," he said in the excerpts. "The United States will do what we must to prevent Iran from obtaining a nuclear weapon."

Romney has sharply criticized Obama for not doing more to force Iran to halt its suspect nuclear program, which the United States and ally Israel believe is an effort to acquire the ability to build a nuclear weapon. The Republican presidential candidate has also said Obama has done too little to help Syrian rebels fighting to overthrow President Bashar Assad.

Obama hit back hard in a "60 Minutes" interview, suggesting that Romney may want to "start another war" in the Middle East.

Ahead of the General Assembly remarks, the Republican National Committee issued a Web video juxtaposing dramatic footage of violent anti-U.S. demonstrations in the Muslim world with Obama's understated observation that there will be "bumps in the road" in the region swept up in the Arab Spring uprisings against authoritarian rule.

Obama and Romney are both scheduled to speak at former president Bill Clinton's Global Initiative on Tuesday. The former Massachusetts governor was to lay out his plans for revamping America's foreign aid.