Iran demanded late Wednesday that world powers sweeten proposals made in talks in Baghdad aimed at easing the crisis over Tehran's nuclear programme, saying future negotiations were at stake.
"The points of agreement are not yet sufficient for another round," an Iranian official said on condition of anonymity and Western officials confirmed the talks would resume at 0500 GMT on Thursday for a second, unscheduled day.
Earlier Wednesday, EU foreign policy chief Catherine Ashton presented a new package of proposals on behalf of the P5+1 -- Britain, China, France, Russia and the United States, plus Germany.
The EU gave no details but they were believed to call on Iran to suspend enrichment of uranium to purities of 20 percent in return for incentives that fell short of meeting Iran's main demand of easing sanctions.
Reflecting official thinking in Tehran, state media ran reports slamming the package, with the IRNA news agency calling it "outdated, not comprehensive, and unbalanced."
Iran made a five-step counter-proposal that an official said was "based on the principles of step-by-step and reciprocity," which the ISNA news agency called "comprehensive... transparent and practical."
"We need the steps that both sides have to take to be clearly defined and there is no possibility of going back on them," the official from the Iranian delegation said.
"For example, that they lift sanctions that they cannot then readopt two months later under a different pretext."
US State Department spokeswoman Victria Nuland told reporters in Washington that the P5+1 package would include "step-by-step reciprocal steps aimed at near-term action on our part if Iran takes its own steps."
Sweeteners reportedly offered included a pledge not to impose any new sanctions, as well easing Iranian access to aircraft parts and a possible suspension of an EU insurance ban on ships carrying Iranian oil.
It also reportedly included a revival of previous attempts to get Iran to ship abroad its stockpiles of enriched uranium in return for fuel for a reactor producing medical isotopes.
Iran says that a lack of fuel plates for this reactor was the reason it started in 2010 to enrich uranium to purities of 20 percent, a capability that reduces the theoretical "breakout" time needed to reach a weapons-grade 90 percent.
Iran announced on Tuesday that it was loading domestically produced, 20-percent enriched uranium fuel into the reactor, and the Iranian official in Baghdad was dismissive.
"A possible swap of uranium enriched by Iran for fuel isn't very interesting for us because we are already producing our own fuel," the Iranian official said.
US President Barack Obama took office in January 2009 offering a radical change in approach to his predecessor, George W. Bush, in dealings with Iran, famously offering an "extended hand" to Tehran if it "unclenched its fist."
This failed, however, and Iran has since dramatically expanded its programme, including by starting in 2010 to enrich uranium to 20 percent and from January in the Fordo site deep inside a mountain near the shrine city of Qom.
Israel, Washington's closest ally in the region, feels its very existence would be under threat if its arch foe gets the bomb and has refused to rule out a pre-emptive strike on Iran's nuclear facilities.
Iran and the major powers returned to talks in Istanbul in mid-April after a 15-month hiatus, finding enough common ground to agree to meet again in Baghdad, hailing what they said was a fresh attitude.
But the Baghdad talks were always going to be tough, as to make progress the two sides would have to tackle some of the thorny issues that have divided them -- and the P5+1 themselves -- for years.
Diplomats and analysts said that a satisfactory outcome would be an agreement to hold more regular talks at working level to thrash out a series of confidence-building measures in what would be a lengthy process.
One key way for Iran to win the confidence of the P5+1 would be to implement the additional protocol of the nuclear Non-Proliferation Treaty, which allows for more intrusive inspections by the International Atomic Energy Agency.
The IAEA also wants Iran to address allegations made in its November report that until 2003, and possibly since, Tehran had a "structured programme" of "activities relevant to the development of a nuclear explosive device."
IAEA chief Yukiya Amano said on Tuesday after talks in Tehran that a deal on ways to go over these accusations with the Iranians would be signed "quite soon." Western reaction though was cool.
"This is the second meeting. Istanbul kicked off the process of discussions. Now we're getting on to the real substance of the matter," Ashton's spokesman Michael Mann said Wednesday.
"We are keen to get a move on but these things can't be solved overnight."