The talks will involve United States, China, Russia, Britain, France and Germany and be led by European Union foreign policy chief Catherine Ashton, her spokesman Michael Mann said yesterday.
US officials confirmed the talks would be held at the level of political directors, with the American side led by seasoned negotiator Under Secretary Wendy Sherman.
State Department deputy spokeswoman Marie Harf said she expected the talks will "probably go until the end of the ministerial, the high-level, week" of the United Nations General Assembly.
The talks are being preceded by smaller bilateral meetings in Geneva and Vienna as the sides discuss how to limit Iran's nuclear enrichment capabilities and number of centrifuges.
Also Read
The West has long accused Tehran of seeking to develop a nuclear bomb, something the Islamic republic has consistently denied, insisting its nuclear program is purely for civilian energy purposes.
Senior US and Iranian officials met yesterday in Geneva for the start of two days of talks, and Mann revealed that France, Britain and Germany would hold a separate round of talks with Iran at the political directors level on September 11 in Vienna.
Quoted by the Belga news agency, Zarif said he was "fairly optimistic" after that group and Iran reach a deal by the November 24 deadline.
But Harf urged Iran to cooperate with UN watchdog the International Atomic Energy Agency (IAEA), after its inspectors were refused access to a military base outside Tehran that they have been trying to visit since 2005.
Iran should work "fully and without delay with the IAEA to resolve all outstanding issues, particularly those that give rise to concerns regarding the possible military dimensions of Iran's nuclear programme," Harf said.