Obama urges diplomatic push on Iran nuclear programme
US President Barack Obama has said recent moves by Iran should offer the basis for a "meaningful agreement" on its nuclear programme.
Speaking at the UN General Assembly's annual meeting, Mr Obama said words now had to be matched by actions.
The US leader recently exchanged letters with his newly-elected counterpart over the nuclear issue.
Later, President Hassan Rouhani insisted Iran posed "absolutely no threat to the world or region".
He said nuclear weapons had "no place in Iran's security and defence doctrine", and denounced international sanctions as "violent, pure and simple".
Earlier, Mr Obama also called for a strong UN resolution on Syria's chemical arms.
He said the purpose of such a resolution should be "to verify that the [Bashar al-Assad's] regime is keeping its commitments" to remove or destroy its chemical weapons.
Mr Obama referred to Iranian suffering from chemical weapons at the hands of Iraq when he said the ban on chemical weapons was "strengthened by the searing memories of soldiers suffocated in the trenches; Jews slaughtered in gas chambers; and Iranians poisoned in the many tens of thousands".
Acknowledging the pain of the other can often be the first step towards some form of reconciliation.
For two countries that have had no diplomatic relations for three decades, Mr Obama's mention of the suffering of Iranians dying from sarin gas attacks in the waning years of the Iran-Iraq war is akin to an olive branch.
Mr Rouhani was not in the UN chambers but his foreign minister was.
The two countries are now engaged in an elaborate diplomatic dance, which will last all week. No-one expects a tangible breakthrough - years of difficult history cannot be overcome overnight, and it's hard to see how Washington and Tehran's regional interests will ever overlap.
But it's the first time since Obama made his offer to engage America's foes during his 2009 inaugural speech that Iran has responded so positively and so publicly.
The deal for Syria to hand over its chemical weapons by mid-2014 was agreed earlier this month between the US and Russia, averting a possible Western military strike.
Differences have since emerged over whether the deal should be enforced by a UN Security Council resolution under Chapter VII of the organisation's charter, which would authorise sanctions and the use of force if Syria did not comply with its obligations.
UN Secretary-General Ban Ki-moon has demanded the international community "bring to justice the perpetrators of the use of chemical weapons in Syria".
Peace 'within reach'
On Iran, Mr Obama said the US wanted to resolve the nuclear issue peacefully, but was determined to prevent Iran from developing a nuclear weapon.
"We are not seeking regime change, and we respect the right of the Iranian people to access peaceful nuclear energy," he insisted - an acknowledgment of the assertion frequently made by Iranian authorities.
"Instead, we insist that the Iranian government meet its responsibilities under the Nuclear Non-Proliferation Treaty and UN Security Council resolutions."
"The roadblocks may prove to be too great, but I firmly believe the diplomatic path must be tested" he added further into the speech, saying he had urged Secretary of State John Kerry to pursue a deal.
Iran insists it is a peaceful programme, but Western countries suspect it of seeking to develop a nuclear weapon.
Mr Rouhani denounced "Iran-phobic discourses" and told the Assembly that Iran sought "constructive engagement", and did not "seek to increase tensions with the United States".
"Our national interests make it imperative that we remove any and all reasonable concerns about Iran's peaceful nuclear programme."
"Let me say loud and clear that peace is within reach," Mr Rouhani said.
Earlier Mr Rouhani shook hands with French President Francois Hollande, who said he expected "concrete gestures" from Iran to show it was not developing nuclear weapons.
But a much-touted informal encounter between Mr Rouhani and Mr Obama failed to materialise, with a senior US administration official saying such a gesture had turned out to be "too complicated for [the Iranians] to do that at this time given their own dynamic back home" - a clear reference, correspondents say, to Iran's Supreme Leader Ayatollah Khamenei.
End Quote Mohammad Javad Zarif Iranian Foreign Minister
We have a historic opportunity to resolve the nuclear issue”
Nonetheless, US-Iran contacts are on the rise. On Thursday, Iran's Foreign Minister Mohammad Javad Zarif will discuss its nuclear programme with Mr Kerry - a rare instance of a formal encounter between the counterparts, say correspondents.
The meeting will be attended by foreign ministers from the other four permanent UN Security Council members - the UK, China, France and Russia - and also Germany, which make up the so-called P5+1.
Mr Rouhani has said he is ready to restart stalled nuclear talks without preconditions - a pledge rubbished by Iranian foe Israel.
"Iran thinks that soothing words and token actions will enable it to continue on its path to the bomb," Israeli Prime Minister Benjamin Netanyahu said in a statement.
Western ministers will want to see an Iranian willingness to make concessions on its nuclear programme if there is to be any lifting or lightening of UN and Western sanctions, BBC diplomatic correspondent Jonathan Marcus says.
Iran for its part will want a clear indication that the US is willing to treat Iran with the respect it believes it deserves as a significant regional player, he adds.
The EU's foreign policy chief, Baroness Catherine Ashton, met Mr Zarif on Monday and described their discussion as "good and constructive." She said her team would hold talks with Mr Zarif again in October in Geneva to assess progress.
Last week, Mr Rouhani said that his country would never "seek weapons of mass destruction, including nuclear weapons", and that his goal was "constructive engagement" with the international community.