The toughest issues must still be negotiated, and any deal could still fall apart.
But Russia, the International Atomic Energy Agency and outside experts tracking the negotiations agree that the Iranians are cooperating, answering questions that they previously avoided and appear to want an agreement that ends crippling sanctions.
Hopes for a deal were bolstered by news that international nuclear inspectors plan to visit two sites in Iran in the coming days, which an official said Sunday would fulfill demands made by the United Nations nuclear watchdog.
A report by Iran's official IRNA news agency quoted Behrouz Kamalvandi, spokesman of Iran's atomic department, as saying the inspectors will visit a uranium mine and a uranium-thickening facility in central Iranian towns of Ardakan and Yazd on Monday and Tuesday.
Daryl Kimball, executive director of the Washington-based Arms Control Association, stressed that negotiators have said repeatedly there's no agreement until all outstanding issues are resolved. But he said he remains optimistic.
"We see encouraging signs that both sides are showing flexibility and pragmatism that is necessary to put together the final package," he said.
Iran insists its nuclear programme is peaceful, aimed at producing nuclear energy, but the US, its Western allies and Israel have long believed Tehran's real aim is producing nuclear weapons.
The experts are meeting in New York this week to prepare for the next ministerial-level negotiations in Vienna on 13 May.
Their closed-door sessions, starting Monday, are taking place on the sidelines of the third and final preparatory conference for next year's review of the landmark 1970 Nuclear Non-proliferation Treaty, a pact aimed at stopping the spread of nuclear weapons.
Patricia Lewis, former head of the United Nations Institute for Disarmament Research who now directs research on international security at London's Royal Institute of International Affairs, said "there's been a big change" in Iran's approach compared to two years ago, "when it was just constant stalemate" on efforts to rein in its nuclear program.
"There has been real moving ahead and real genuine progress in what they've been trying to achieve," she said. "They're small steps, but they're significant steps. I would say, so far, so good. I'm holding my breath."
The change in Iran was sparked by the election of centrist President Hasan Rouhani, who replaced hard-liner Mahmoud Ahmadinejad in June 2013. Rouhani pledged to reduce confrontation with the international community over its nuclear activities, and agreed to restart stalled negotiations with the six powers in November.
That agreement with the US, Russia, China, Britain, France and Germany requires Iran to stop enrichment of uranium to 20% — which is a possible pathway to nuclear arms — in exchange for the easing of some Western sanctions.
The two sides are working toward a permanent agreement by July 20 that would cap Iran's enrichment program, reduce its stockpile of 20% enriched uranium and turn some of it into a solid form which is less usable in bomb-making, and curb other atomic activities in exchange for lifting all sanctions.
The November agreement also agreed to allow the UN nuclear watchdog, the International Atomic Energy Agency, to visit its nuclear facilities.
When the last round of high-level negotiations ended on 9 April, Iranian Foreign Minister Mohammad Javad Zarif said his country and the six powers were in "50 to 60% agreement" on the shape of a deal.
Since those high-level talks, the IAEA reported that Iran has been complying with all its requests. US State Department spokesperson Marie Harf also reported in mid-April that "all sides have kept the commitments" they signed on to.
Kimball said the toughest issues for the experts next week include trying to agree on how many centrifuges Iran will be allowed to keep to enrich uranium, and in what time period, and the sequence and pacing of sanctions relief.
Lewis said Iran is going to be allowed to enrich uranium to 5% and not beyond, which is what most countries do. But Tehran has a research reactor that provides medical isotopes and requires 20%-enriched uranium, so that's a big issue.
She said another big issue is deciding what to do about Iran's heavy-water reactor at Arak, which is designed to produce plutonium, another key ingredient for making nuclear weapons.
Iran announced on April 19 that it will redesign the Arak reactor to greatly limit the amount of plutonium it can make.
Lewis, a nuclear physicist, said phasing out sanctions is controversial, particularly in the United States, and there must be fallback positions in case things don't go well.
Mikhail Uliyanov, director of the Russian Foreign Ministry's Department of Arms Control and Nonproliferation, told the NPT preparatory conference on Tuesday that Iran's "unprecedentedly constructive co-operation" with the IAEA and the six powers "gives grounds to hope for a successful outcome of the talks".