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 that are transparent and verifiable”.
The US leader recently exchanged letters with his newly-elected counterpart over the nuclear issue.
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.
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.
Opening the UN summit on Tuesday, Secretary-General Ban Ki-moon said the Syrian government must “fully and quickly honour” its obligations under the deal.
“The international community must bring to justice the perpetrators of the use of chemical weapons in Syria, confirmed unequivocally by the UN investigation mission,” he said.
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.
“The roadblocks may prove to be too great, but I firmly believe the diplomatic path must be tested” he said, adding that he had urged his Secretary of State John Kerry to pursue a deal.
“Iran’s genuine commitment to go down a different path will be good for the region and the world,” Mr Obama said.
Iran insists it is a peaceful programme, but Western countries suspect it of seeking to develop a nuclear weapon.
The country’s new President, Hassan Rouhani, has said he wants to present his country’s “true face”.
A meeting between Mr Obama and Mr Rouhani – the first such encounter since the 1979 revolution – has not been ruled out.
There is also speculation that he and Mr Obama may shake hands on the sidelines of the General Assembly.
On Thursday, Iran’s Foreign Minister Mohammad Javad Zarif will discuss its nuclear programme with Mr Kerry and other diplomats.
A foreign ministry spokeswoman in Tehran said the meeting represented the “beginning for nuclear talks in the new era”.
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.
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.