Iran gave no sign of making concessions to world powers bent on coaxing it to curb its nuclear program on Friday, saying it would not discuss suspending sensitive uranium enrichment.
The West suspects Iran plans to develop a nuclear weapon but Tehran says its atomic energy program is peaceful.
The Istanbul meeting is a follow-up to one held last month in Geneva, which was the first in more than a year.
At the outset, an aide to the Islamic Republic’s nuclear negotiator Saeed Jalili drew a red line round its enrichment activities during the meeting.
“We will not allow any talks linked to freezing or suspending Iran’s enrichment activities to be discussed at the meeting in Istanbul,” Abolfazl Zohrevand told reporters.
Uranium enriched to a low degree yields fuel for electricity or, if refined to a very high level, the fissile core of a nuclear bomb.
Prospects for progress were further thrown in doubt by uncertainty over whether Iran would agree to a bilateral meeting between Jalili and a U.S. delegation led by Under-Secretary for Political Affairs Bill Burns.
The meeting was requested by European Union foreign policy chief Catherine Ashton as part of a series of bilaterals between Iran and the six big powers.