The foreign ministers of arch-rivals, the United States and Iran, will hold their first talks on Iran's contested nuclear drive at a landmark meeting on Thursday, officials said.
Mohammad Javad Zarif, foreign minister in Iran's new government, and US Secretary of State John Kerry, will join counterparts from Britain, China, France, Germany and Russia at the meeting at United Nations headquarters, according to US officials.
High level contacts between Iranian and US officials have been rare since the 1979 Iranian Revolution. But in a new sign of a possible thaw, the White House said it was not ruling out a meeting between President Barack Obama and new Iranian President Hassan Rowhani on the sidelines of this week's UN General Assembly.
The encounter between Zarif and Kerry, confirmed by US officials -- and arranged for Thursday, according to diplomats -- comes as Iran signals it wants the international community to ease crippling sanctions over its uranium enrichment.
Rowhani said in a US television interview last week that Iran would "never" build a nuclear bomb. But the United States and its allies still believe Tehran wants that capability and are waiting for signs that Rowhani is serious about seeking better relations, diplomats said.
Thursday's talks "will give our ministers a sense of their level of seriousness and whether they are coming with concrete new proposals and whether this charm offensive actually has substance to it," a senior State Department official said.
The P5+1, as the group is known, made a revised proposal to Tehran earlier this year, before Rowhani's election, believed to offer some relief on international sanctions in return for a scaling back of the country's uranium enrichment program.
"There's a sense that we never actually got a firm response or a detailed response to that," the US official said.
"So this ... first session at the level of ministers where the Iranians will join a meeting already in progress will give a sense of the road ahead with the new government."
"But this is just the start, and that would obviously, we would hope, include a real response to the proposal that's been on the table for some time," the official added.
Another State Department official stressed, however, that "no-one should have the expectation that we're going to resolve this decades-long issue during the P5+1 meeting later this week," adding that the ball was firmly in Iran's court.
Zarif met some Western officials in New York on Monday, ahead of the arrival of Rowhani to address the UN General Assembly on Tuesday.
Catherine Ashton, the European Union's foreign policy chief and the leader of efforts by the major powers to engage with Tehran, said she had "good and constructive" talks with Zarif.
She voiced hope for progress, but played down hopes of a breakthrough.
"I was struck by the energy and determination on the part of the minister," Ashton told reporters. But she added: "As you would appreciate, there is a huge amount of work to do."
Thursday's meeting will be a milestone between the United States and Iran, which broke off diplomatic relations in 1980 after the overthrow of the late Shah and the taking of US diplomats as hostages.
In 2007, US secretary of state Condoleezza Rice met her Iranian counterpart informally at an international summit in Sharm El-Sheikh, Egypt.
Her predecessor, Colin Powell, met Iran's foreign minister at the same venue in 2004. But no Iranian minister has taken part in ministerial meetings of the major powers trying to persuade Iran to halt its nuclear drive.
Ben Rhodes, a deputy national security adviser, told reporters that while Obama was not scheduled to meet with Rowhani, it could not be ruled out.
"We are open to engagement with the Iranian government on a variety of levels provided that they will follow through on their commitment to address the international community's concern on their nuclear program," he said.
In an opinion piece in the British daily The Guardian, former Iranian president Mohammad Khatami warned against letting diplomacy fail, singling out Israel for "a campaign to discredit" Rowhani who he said enjoyed the "explicit support" of Iran's supreme leader.
"Failure now to create an atmosphere of trust and meaningful dialogue will only boost extremist forces on all sides," Khatami wrote.
Israeli Prime Minister Benjamin Netanyahu has criticized overtures to Rowhani, calling him a "wolf in sheep's clothing" and urging no let-up in pressure.
But Greg Thielmann, a senior fellow at the Washington-based Arms Control Association, saw potential signs of progress in jumpstarting diplomacy on Iran.
"You've got all these signals from the Iranians that they are willing and trying to be more constructive," Thielmann said.
Rowhani will address the General Assembly on Tuesday after meeting French President Francois Hollande.