But the six powers known as the P5+1 (the five permanent members of the UN Security Council plus Germany) had yet to agree on a United Nations Security Council resolution that would lift UN sanctions and establish a means of re-imposing them in case of Iranian non-compliance with a future agreement.
"We still haven't sorted a Security Council resolution," a diplomat close to the talks told Reuters. "We don't have Iran on board yet."
Senior diplomats echoed the remarks. Some of the toughest disputes, including the question of easing UN sanctions, were likely to be left for foreign ministers, officials said.
"Even if and when issues get resolved at an experts level, there will remain some open issues that can only be decided by ministers," a senior US official told reporters. Iran is in talks with the United States, Britain, China, France, Germany and Russia on an agreement which would limit its nuclear program for a specified period of time in exchange for relief from economic sanctions. The negotiators missed a June 30 deadline for a final agreement, but have given themselves until July 7. Foreign ministers not in Vienna were expected to rejoin their counterparts in a final push for a deal beginning on Sunday.
Iran issued a warning about consequences of a collapse in the talks.
"The other side has tried all kinds of pressure against Iran and if it intends to test these pressures again, as (President Hassan) Rouhani has said, Iran's response would be actions harder than what the other side imagines," said spokesperson for the Atomic Energy Organization of Iran Behrouz Kamalvandi.
All sides say a deal is within reach. But there are other sticking points in addition to sanctions and future monitoring mechanisms.
One is a UN investigation into what it calls the possible military dimensions of past Iranian nuclear research. Tehran denies its nuclear program may have any military objectives, saying the work is solely for peaceful purposes.
Another issue is Iran's demand to continue research and development work on advanced uranium enrichment machines.
Foreign Minister Mohammad Javad Zarif, US Secretary of State John Kerry and Chinese Foreign Minister Wang Yi had remained in Vienna. Senior officials from Iran and the six powers have continued meetings to try to finalize an agreement.
Signs of Compromise
Some officials said there were signs of a compromise emerging on one of the major sticking points: access to Iranian sites to monitor compliance with a future agreement.
Another potential emerging compromise relates to Iran's stockpile of low-enriched uranium. Diplomats claimed Tehran was considering shipping most of the stockpile out of the country.
Regarding the PMD issue, western diplomats said the International Atomic Energy Agency had to be satisfied it knew the full scope of past Iranian activity to establish a credible basis for future monitoring.
Meanwhile, CNN quoted a source familiar with the talks as saying experts from the countries involved in the Iran nuclear negotiations have reached a tentative agreement on language dealing with sanctions relief.
However, the source said there is a lot of work to do and this language still has to be presented to the foreign ministers of the negotiating parties, meaning this is far from complete.
"We have resolved many problems so far, but there are still questions to be tackled on nuclear as well as sanctions issues," a senior Iranian diplomat directly involved in the nuclear talks told CNN.
Earlier, a US administration official told the pool traveling with Kerry that, "Even if and when issues get resolved at an experts level, there will remain some open issues that can only be decided by ministers."
A source directly involved in the talks said the sides would declare at a deal signing that sanctions are lifted, but the sanctions would only be removed over time as Iran meets its obligations.
This would come in addition to certain funds being released right away as part of the so-called "signing bonus," the source added.
