The US and its European allies have hailed a new tone and a significant shift in attitude from Iran in talks aimed at resolving the impasse over the country's disputed nuclear activities.
Iranian foreign minister Mohammad Javad Zarif, who also had an unexpected one-to-one meeting with US secretary of state John Kerry at the United Nations, said six world powers and Iran had agreed to fast-track nuclear negotiations, with the hope of reaching a deal within a year.
Iran said it was eager to dispel suspicions that it was trying to develop a nuclear weapon and to get punishing international sanctions lifted as fast as possible.
Iran, the five permanent members of the UN Security Council and Germany also agreed to hold a new round of substantive nuclear negotiations on October 15 and 16 in Geneva, Switzerland.
"We agreed to jump-start the process so that we could move forward with a view to agreeing first on the parameters of the end game ... and move towards finalising it hopefully within a year's time," Mr Zarif said after the talks in New York ended.
"I thought I was too ambitious, bordering on naivete. But I saw that some of my colleagues were even more ambitious and wanted to do it faster."
Mr Kerry said he was struck by a "very different tone" from Tehran after their sessions, which marked the highest-level direct contact between the United States and Iran in six years.
But, like his European colleagues, he stressed that a single meeting was not enough to assuage international concerns that Iran is seeking to develop nuclear weapons under cover of a civilian atomic energy programme.
"Needless to say, one meeting and a change in tone, that was welcome, does not answer those questions," he told reporters late last night. "All of us were pleased that the foreign minister came today and that he did put some possibilities on the table."
He said they agreed to continue the process and try to find concrete ways to answer the questions that people had about Iran's nuclear activities.
Mr Zarif and Mr Kerry sat next to each other at a U-shaped table during the group talks. It was the highest-level direct contact between the United States and Iran in six years. EU foreign policy chief Catherine Ashton suggested the two men had shaken hands and been cordial with each other.
She also said the parties had agreed to "go forward with an ambitious timeframe".
Mr Zarif said the meetings were "very constructive" and "very substantive".
"We hope to be able to make progress to solve this issue in a timely fashion (and) to make sure (there is) no concern that Iran's programme is anything but peaceful," he said.
"I am satisfied with this first step. Now we have to see whether we can match our positive words with serious deeds so we can move forward."
He said the end result would have to include "a total lifting" of the international sanctions that have devastated Iran's economy.
Britain's Foreign Secretary William Hague said there had been a "big improvement in the tone and spirit" from Iran compared with the previous government under Mahmoud Ahmadinejad.
Iranian president Hassan Rouhani and Mr Zarif, both in New York for the UN General Assembly, have said they are anxious to clinch an agreement quickly that could bring relief from sanctions that have slashed the country's vital oil exports, restricted its international bank transfers, devalued the currency and sent inflation surging.
Mr Rouhani has come across as a more moderate face of the hardline clerical regime in Tehran and his pronouncements at the UN have raised guarded hopes that progress might be possible. But they have also served as a reminder that the path to that progress will not be quick or easy.
In his speech to world leaders at the UN on Tuesday, he repeated Iran's long-standing demand that any nuclear agreement must recognise the country's right under international treaties to continue enriching uranium.
The US and its allies have long demanded a halt to enrichment, fearing Tehran could secretly build nuclear warheads and have imposed sanctions over Iran's refusal to halt enrichment. Uranium enriched to low levels can be used as fuel for nuclear energy but at higher levels, it can be used to make a nuclear weapon.
Mr Rouhani also insisted that any deal be contingent on all other nations declaring their nuclear programmes, too, are solely for peaceful purposes - alluding to the US and Israel.
Those conditions underscore that there is still a large chasm to be bridged in negotiations.
Mr Rouhani has made a series of appearances and speeches since arriving in New York and has held bilateral negotiations with France, Turkey and Japan among others.
Yesterday he called for worldwide disarmament of nuclear weapons as "our highest priority" and r epeated the organisation's long-standing demand that Israel join the international treaty banning the spread of nuclear weapons.
Israel, which has repeatedly accused Iran of aspiring to build a nuclear bomb, is the only Middle East state that has not signed the landmark 1979 Non-Proliferation Treaty.
Mr Rouhani appears to be trying to tone down Mr Ahmadinejad's caustic rhetoric against Israel - a point of friction in relations with the US. But Israel reacted angrily to his latest remarks.
"The problem of the NPT in the Middle East is not with those countries which have not signed the NPT, but countries like Iran, Iraq, Libya and Syria which have signed the treaty and brazenly violated it," said intelligence and international affairs minister Yuval Steinitz.
"Unlike Iran, Israel has never threatened the destruction of another country."