Obama to pledge full US role in Mideast President Barack Obama will tell the United Nations on Tuesday that the US will not shrink from its role in the fast-evolving Muslim world and will "do what we must" to prevent Iran becoming a nuclear military power. In a speech to the annual general assembly of the UN, the president will hit back at accusations by critics, including Mitt Romney the Republican candidate for the presidency, that he has been weak in the face of rage against the US following the release of an inflammatory video apparently mocking the prophet Mohammad. According to excerpts from his speech released by the White House, Mr Obama will address the attack on the US consulate in Benghazi, which left the US ambassador Chris Stevens and three other Americans dead. "We must affirm that our future will be determined by people like Chris Stevens, and not by his killers," Mr Obama will say, according to the White House. There are no words that excuse the killing of innocents. There is no video that justifies an attack on an embassy. He will tell Muslim countries that the world faces a choice "between the forces that would drive us apart and the hopes we hold in common." The speech, scheduled for 10am EST (3pm BST), adds: "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. Mr Obama will also, according to a White House preview, say in a television interview due to be aired on ABC on Tuesday that the attacks in Benghazi "wasn't just a mob action." Mr Obama will also address another area on which he has been attacked by his rival for the Oval Office. Mr Romney has accused him of adopting a weak stance on Iran's efforts to get nuclear weapons as well as neglecting US relations with its ally Israel. Mr Obama will say: "Make no mistake: a nuclear-armed Iran is not a challenge that can be contained." His speech adds: "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. Commentators have noted that Mr Obama has not arranged any bilateral meetings at the general assembly, returning straight to campaigning after his speech.