"We had hoped to have a detailed and constructive discussion of those ideas," Ashton said. "But it became clear the Iranian side was not ready for this unless we agreed to preconditions regarding enrichment and sanctions."
The negotiations resumed Saturday morning after an opening session that lasted almost until midnight.
During what appears to have at times been a contentious day of talks, Western officials said the Iranian delegation tested the unity of P5+1.
"She was very solid. She didn't fall into any of their traps," said one Western diplomat, speaking on condition of anonymity late Friday night.
The diplomat claimed that the chief Iranian negotiator, Saeed Jalili, "realized that the six are solid and that the six mean business."
Before the discussions even began in a 19th century Ottoman palace overlooking the waters of the Bospohorus Strait, Iran officials declared that they would not allow any discussion of suspending Iran's controversial uranium enrichment program.
And during a mid-day break on Friday that involved a trip across town to pray at Istanbul's famous Blue Mosque, a member of the Iranian delegation repeated this tough negotiating position.
"We will not allow any talks on the halt or suspension of uranium enrichment activities in Iran to be raised in the Istanbul talks," said Abolfazi Zohrehvand, a spokesman for Iran's Supreme National Security Council, according to Iran's Fars News Agency.
"There has been no word on enrichment at the talks at all and our country's nuclear rights have not been an issue on debate either," he added.
Another Western official says the P5+1 countries "summarily rejected" the Iranian delegations demands at the outset of the talks to establish "preconditions" which included accepting a full nuclear fuel cycle for Iran and the removal of sanctions.
Last year, the United Nations Security Council imposed a fourth round of sanctions on Iran as punishment for its "lack of compliance ... on ensuring the peaceful nature of its nuclear programme."
Iran insists its nuclear facilities are working to develop technology for civilian energy needs.
Analysts have wondered what Iran wants to accomplish with the talks.
"Iran wants to widen the nuclear discussions to disarmament and to the case of Israel, and does not appear to be willing to talk about the Iranian nuclear dossier which originally prompted the P5 plus 1 process," wrote Ollie Heinonen in an e-mail to CNN.
Until last August, Heinonen spent the last five years leading efforts at the UN's International Atomic Energy Agency to contain Iran's nuclear program.
"I really hope that the discussions continue in near future since slowing down of the enrichment program has created a window of opportunity which should not be missed," he added.
But another group that has been studying Iran's nuclear program, accused Tehran of playing for time as it continues enriching uranium.
"Protracted talks are clearly in Tehran's interest," Ivanka Barzashka of the Federation of American Scientist wrote in an e-mail to CNN. "As it steadily chugs forward with its enrichment program, it increases its potential to build a nuclear bomb."
The Iranian government has repeatedly denied accusations that it is trying to develop nuclear weapons technology.
