Iran and six world powers are in "50 to 60% agreement" on the shape of a nuclear deal meant to crimp any potential Iranian attempt to build nuclear arms in exchange for an end to crippling economic sanctions, the country's foreign minister said.
Speaking for the six, EU foreign policy chief Catherine Ashton was less upbeat as the talks reached the half-way mark toward their informal July deadline. But she said that after several rounds of exploratory talks the two sides were now ready to bridge remaining gaps standing in the way of agreement.
The talks paused until May 13 amid stern warnings from Iran's supreme leader, whose message has varied over the past months between support for the discussions and accusations of bad-faith negotiating on the part of the United States and its allies.
"Our negotiators should not accept any coercive words from the other party," the country's supreme leader Ali Khamenei told Iranian nuclear scientists in a speech marking Iran's National Day of Nuclear Technology. "The country's nuclear achievements can't be stopped, and no one has the right to bargain over it."
Coming out of the Vienna talks, Ms Ashton described them as "substantive and detailed."
She said that "a lot of intensive work will be required to overcome the differences ahead".
But Ms Ashton suggested some progress was made and negotiators are now looking to the next round to "bridge the gaps standing in way of a comprehensive agreement."
Iranian foreign minister Mohammad Javad Zarif, his country's chief negotiator, read the same statement in Farsi. He then told reporters that the two sides were in "50 to 60% agreement", adding he expected the July 20 target for a final deal to be met.
A senior US administration official declined to confirm whether that assessment was accurate but said only 100% agreement by both sides would produce a deal.
Unlike previous rounds the May meeting will be open-ended to allow negotiators to meet all week if needed as they increase efforts to seal a deal, she said.
The world powers are offering to remove sanctions that have crippled Iran's economy provided Tehran agrees to strict long-term limits on any nuclear activities that could be used to make a weapon.
The future scope of Iran's uranium enrichment programme is the toughest issue.
Iran denies any interest in nuclear arms and argues it needs robust enrichment capacities to make low-enriched reactor fuel. The US, Britain, France and Germany want significant cuts, to decrease the chances that the programme will be re-engineered to make high-enriched material for atomic arms. Russia and China are somewhere in the middle.
The six also want to eliminate potential proliferation dangers from an enrichment site at Fordo, south of Tehran, that is built far underground to withstand air strikes, and at a nearly built nuclear reactor at Arak, in northwestern Iran, that could produce substantial amounts of plutonium unless it is changed to a model with new specifications.
Like enriched uranium, plutonium can be used for the core of a nuclear weapon.
A first-step deal, in effect since January, has curbed some Iranian nuclear activities in return for limited sanctions relief as the two sides work toward a final agreement. Iranian hardliners fear that end deal will cut too deeply into their country's nuclear programme, a source of national pride.
In Tehran, Mr Khamenei sought to dispel such fears.
"These talks need to continue," he said. "But all must know that despite continuation of the talks, activities of the Islamic Republic of Iran in the field of nuclear research and development won't be halted at all."