World powers began a final day of talks with Iran on its nuclear programme on Saturday with little sign of any headway that could rescue the Istanbul meeting from failure.
Progress on the first day was thin, with Iran insisting on preconditions, including the lifting of sanctions and recognition of its right to enrich uranium.
It blamed the big powers -- the United States, Britain, France, Russia, China and Germany -- for the impasse.
"The outcome of these talks should pave the ground for the next talks ... Unfortunately the other party has no clear and convincing agenda for talks and they just want to hold talks," said Abolfazl Zohrevand, an aide to Iran's chief negotiator Saeed Jalili.
"Today we want some tangible ways to be found for active cooperation in various fields."
The West suspects Iran plans to develop nuclear weapons while Tehran says its atomic energy programme is peaceful. The standoff has dragged on for eight years.
A Western diplomat at the talks said the powers hoped for signs that Iran wanted to engage seriously this time, rather than set unacceptable preconditions.
"Iran's preconditions -- asking us to accept Iran having a full fuel cycle and lift sanctions upfront before any progress -- were summarily rejected," he said.
"Today, we'll see if they are ready to get serious."
Negotiators went into the Istanbul meeting with low expectations of a breakthrough in a meeting that is a follow-up to one held in Geneva last month, which was the first time the two sides had met in more than a year.
On Saturday, Iran said it had rejected a proposal for a bilateral meeting with the United States, saying it was not necessary within the context of the Istanbul nuclear talks.
"But there is no need to hold such talks if it is not about the nuclear meeting in Istanbul," Zohrevand told Iran's Arabic-language Al Alam television news channel.
Since the fall of the US backed shah in 1979, any contacts between the Islamic Republic and the United States have usually taken place behind the scenes, and were rarely confirmed by the Iranian side.
"Differences still there"
The six powers, whose delegations were led by European Union foreign policy chief Catherine Ashton, had braced for a possible collapse in talks which ran late into the night on Friday.
"The differences are still there," said another Western diplomat at the meeting. "But I think the Iranians were a bit taken aback by the unity of the six."
The talks were due to finish by Saturday afternoon, following a meeting between Iran and the Vienna Group, which includes the United States, France and Russia, and a plenary session.
Iran has ignored UN Security Council resolutions demanding it suspend enrichment, with trade and other benefits offered in return, and refused to grant unfettered access for UN nuclear inspectors.
Uranium enriched to a low degree yields fuel for electricity or, if refined to a high level, the fissile core of a nuclear bomb.
Iran's nuclear standoff with the West has escalated in the past year, with the United Nations imposing new sanctions and Western states rejecting a revised proposal for Iran to swap some of its fuel abroad as too little, too late.
Ashton outlined a possible revised offer for a nuclear fuel swap that would entail Iran handing over a large chunk of its stockpile of low enriched uranium.
No offer was made as Iran's preconditions included the suspension of economic sanctions, a Western diplomat said.
The powers want to prevent Tehran from accumulating enough for a nuclear weapon while negotiations proceed on a broader solution to the crisis.