US President Barack Obama addresses the 67th United Nations General Assembly at the UN headquarters in New York
US President Barack Obama has told the UN General Assembly in New York that the US will "do what we must" to stop Iran acquiring nuclear weapons.
Six weeks before the US election, the US president said a nuclear-armed Iran "is not a challenge that can be contained".
Obama condemned the violence that erupted over a "disgusting" anti-Islam video as "an attack on UN ideals".
Unrest across the Middle East is set to dominate discussion the summit, reports the BBC.
Recent protests across the Muslim world in response to the US-made video mocking the Prophet Muhammad, as well as Iran's nuclear programme and the 18-month conflict in Syria, are likely to be high on the agenda.
Opening the meeting on Tuesday, UN Secretary General Ban Ki-moon described the fighting in Syria as "a regional calamity with global ramifications".
He called for action from the divided UN Security Council and said "the international community should not look the other way as violence spirals out of control".
"Brutal human rights abuses continue to be committed, mainly by the government but also by opposition forces," he added.
People did not look to the UN to be simply a mirror reflecting back a divided world, said Ban: Rather, they wanted to see it come up with solutions to problems.
The US president was blunter in his assessment of Syria, saying Bashar Assad's regime must end.
Obama opened his address with a tribute to the US ambassador to Libya murdered in Benghazi, challenging the UN to affirm that "our future will be determined by people like Christopher Stevens, and not by his killers".
"Today, we must declare that this violence and intolerance has no place among our United Nations," he said.
"The United States will do what we must to prevent Iran from obtaining a nuclear weapon," he said, with the backing of "a coalition of countries" holding Tehran accountable.
Although the White House said the president's address should not be considered a campaign speech, it follows critical remarks about his foreign policy from Republican presidential candidate Mitt Romney.
His presidential rival condemned Obama's description of the murder of Stevens and three other Americans as "bumps in the road". He has also castigated him for not taking time out to hold talks on Iran during the summit with Israeli PM Benjamin Netanyahu.
Obama has rejected the Israeli leader's calls for Washington to set Tehran "red lines".
Netanyahu has recently appeared on US television to press for a tougher line on Iran, and he will take the same message to the General Assembly on Thursday.
Tehran says its nuclear programme is for civilian purposes.
On the eve of the assembly, Iranian President Mahmoud Ahmadinejad told a UN meeting that Israel was a "fake regime", prompting Israel's UN ambassador, Ron Prosor, to walk out.
Syria's 18-month conflict is not formally on the General Assembly's agenda but was a focal point of discussion on the opening day, with further comments expected from leaders including French President Francois Hollande and Qatari emir Sheikh Hamad bin Khalifa al-Thani.
Hollande, in his first appearance at the assembly, is also expected to call for backing for an international force to be sent to the West African state of Mali to help dislodge Islamist militants who have taken over the north of the country.
The UN Security Council has been unable to reach agreement on the Syria crisis and on Monday UN envoy Lakhdar Brahimi warned that the situation was "extremely bad and getting worse".
While he did not have a full plan, he said he had "a few ideas". Brahimi has just visited Damascus as well as refugee camps in neighbouring Jordan and Turkey.
The BBC's Barbara Plett says that diplomats have played down expectations for Brahimi's mission, with no sign of fundamental divisions on the council being bridged.