US President Barack Obama has said that he has spoken over the phone with Iranian President Hassan Rouhani.

He said both men had directed their teams to work expeditiously toward an agreement on Iran's nuclear program.

The call was the highest level contact between the United States and Iran since 1979, Mr Obama noted.

He said this was an unique opportunity to make progress with Iran over an issue that has isolated it from the West.

Earlier, Iran and the United Nations nuclear agency held "constructive" talks about Iran's disputed atomic programme and said they would meet again on 28 October.

Deputy Director General of the International Atomic Energy Agency  Herman Nackaerts said the discussions had been "very constructive" but gave no details.

Iran's representative, Ambassador Reza Najafi, said the aim was to reach an agreement "as soon as possible".

The IAEA wants to resume an investigation into suspected atomic bomb research by Tehran, which denies the charge.

The meeting was the first since new Iranian President Hassan Rouhani took office last month pledging to try to resolve the Islamic state's nuclear dispute with the West.

It came after talks between Iran and world powers at the UN yesterday.

Iran and the US held their highest-level substantive talks in a generation at the UN, saying the tone was positive but sounding cautious about resolving the standoff.

US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif met after Mr Zarif held wider talks with the US and other major powers to address Western suspicions that Iran may be trying to develop atomic weapons.