United Nations: President Barack Obama Tuesday vowed to hunt those behind the "attack on America" in Libya that killed the US ambassador and said a "disgusting" film that insulted Muslims was no excuse for violence.

"The attacks on our civilians in Benghazi were attacks on America. There should be no doubt that we will be relentless in tracking down the killers and bringing them to justice," Obama told the UN General Assembly.

But while eulogizing US ambassador Christopher Stevens, who was killed two weeks ago along with three other Americans, Obama also styled the attack as not just an assault on America, but also on the ideals behind the United Nations.

"Today, we must affirm that our future will be determined by people like Chris Stevens, and not by his killers. Today, we must declare that this violence and intolerance has no place among our United Nations."

In his annual speech to the world body, Obama again condemned the video produced by Coptic Christian extremists in the United States that set violence raging across the Arab world as "crude and disgusting."

But he said that however vile, no exercise of free speech that is protected by the US Constitution could justify killing and violence.

"Here in the United States, countless publications provoke offense. Like me, the majority of Americans are Christian, and yet we do not ban blasphemy against our most sacred beliefs.

"Moreover, as president of our country, and commander-in-chief of our military, I accept that people are going to call me awful things every day, and I will always defend their right to do so."

Obama also warned that in 2012 "when anyone with a cell phone can spread offensive views around the world with the click of a button, the notion that we can control the flow of information is obsolete."

"The question, then, is how we respond. And on this we must agree: there is no speech that justifies mindless violence."

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