UN is an international organisation founded after World War II aims to provide a dialogue platform between countries, fortifying international corporation and preventing future wars. There are currently 193 members lead by five permanent members of the Security Council - US, Russia, UK, France, and China. The organization currently has six principal organs: General Assembly, Security Council, Economic and Social Council, Secretariat, International Court of Justice, and the Trusteeship Council.
High drama as stage set for US-Iran nuclear talks
John Kerry and his Iranian counterpart sit down to try to resolve stand-off, but progress is expected to be slow, difficult and fragile
Reuters in New York and Washington
The United States and Iran have set the stage for what could become their most serious contacts in a generation, but direct talks on the Iranian nuclear programme are likely to be slow, difficult and fragile.
Years of sporadic negotiations between Tehran and world powers have failed to yield a deal for curbs on Iran's nuclear programme in return for relief from stiff international sanctions.
The talks are due to resume today, attended by US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif against a backdrop of rare conciliatory gestures between the US and Iran.
President Barack Obama, in his speech to the UN General Assembly on Tuesday, made several gestures to Washington's long-time adversary, including accepting Iran's right to the peaceful use of nuclear energy.
Still, he acknowledged, "the roadblocks may prove to be too great" to secure a diplomatic deal.
In his speech to the assembly, Rowhani, while denying Iran was seeking a nuclear weapon, said he was prepared to engage in "time-bound and results-oriented" nuclear talks.
He pleaded for "a consistent voice from Washington", and said he hoped Obama would not be swayed by "war-mongering pressure groups".
Whether the diplomatic signalling translates into flexibility at the negotiating table won't be known for some time. "After the initial excitement of a change in atmosphere, I expect each side to move slowly," said Jon Alterman, director of Middle East programmes at the Centre for Strategic and International Studies in Washington.
"This is in part because there are politicians on each side who think rapprochement is a strategic mistake, and in part because negotiators fear haste will push one into making a deal harmful to one's interests. In fact, it would surprise me if there isn't a crisis in negotiations as each side tests the other's breaking point."
Formally on the table is a standing offer from world powers for Iran to suspend its most sensitive uranium-enrichment work in return for modest relief from sanctions. Iran did not accept that offer during the most recent round of talks in April in Kazakhstan.
The talks take place in a diplomatic format known as the P5+1, the five permanent members of the Security Council and Germany.
Richard Haass, a former White House and State Department official who is now president of the US Council on Foreign Relations, said negotiators faced technical and political challenges.
He said negotiators must agree on how much nuclear activity Iran can conduct, how nuclear inspections would be conducted and what sanctions relief Iran could expect.
Rowhani, elected on a mandate to improve Iran's economy, made clear in his UN speech that his priority is the lifting of the sanctions that have cut deeply into Iran's oil income. International sanctions against Iran "are violent, pure and simple", he said.
Yet nitty-gritty details may be the relatively easy part. Any deal - and the potential US-Iranian rapprochement it might bring - faces scepticism if not outright hostility from many sources, including Israel, the US Congress and hardliners in Iran.
"Both sides have what political scientists would call a domestic management challenge," Haass said.
In Washington, some lawmakers of both parties favour tightening, not easing, the sanctions on Iran. Republicans John McCain, Lindsey Graham and Kelly Ayotte say Kerry should press Rowhani on issues well beyond the nuclear file. They cited Iran's support for violent militant groups, as well as human rights abuses and backing for Syrian President Bashar al-Assad.
Rowhani may also be hemmed in.
Esmail Kowsari, a conservative member of parliament, who sits on the Majlis' National Security and Foreign Policy committee, has said "negotiations with the United States will not happen". Supreme Leader Ali Khamenei "will never give a green light" for direct talks.