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, BBC reports.Six weeks before the US election, the US president said a nuclear-armed Iran “is not a challenge that can be contained”. Mr Obama condemned the violence that erupted over a “disgusting” anti-Islam video as “an attack on UN ideals”.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 Mr 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.Mr 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”.