Nuclear experts from Iran and six world powers are meeting in New York this week amid rising hopes that an agreement can be reached to limit Iran's ability to produce nuclear weapons before a July 20 deadline.
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. Behrouz Kamalvandi, spokesman of Iran's atomic department, said the inspectors will visit a uranium mine and a uranium-thickening facility in the central Iranian towns of Ardakan and Yazd on Monday and Tuesday, Iran's official IRNA news agency reported.
Daryl Kimball, executive director of the Washington-based Arms Control Association, stressed that negotiators have said repeatedly there is no agreement until all outstanding issues are resolved. But he said he remained 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 program is peaceful, aimed at producing nuclear energy, but the U.S., 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 May 13.
Their closed-door sessions are taking place on the sidelines of the third and final preparatory conference for next year's review of the landmark 1970 Nuclear Nonproliferation Treaty, a pact aimed at stopping the spread of nuclear weapons.
Iran's UN Mission said Monday that the two-day experts meeting will begin on Tuesday. The Iranian delegation will be led by Hamid Baeidinejad, the Foreign Ministry's director of political and international security affairs.
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 has 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.