Iran's ambassador to the International Atomic Energy Agency (IAEA), Ali-Asghar Soltanieh, who heads the Iranian delegation, said on Sunday that the third round of negotiations in the Iranian capital was likely to last for three days.
The second round of talks between IAEA experts and Iranian representatives ended in Tehran on April 30 with no comment from the parties involved.
The IAEA said in a report presented in March that Iran had become more transparent on its nuclear program, but had failed to fully clarify issues relating to allegations of nuclear weapons development by Western nations.
The international community has demanded that Tehran halt uranium enrichment, which can be used both in electricity generation and nuclear weapons production. Iran insists on its right to civilian nuclear energy, and has defied three sets of United Nations sanctions over its nuclear program.