But Kerry added, “Needless to say, one meeting and a change in tone, which was welcome, doesn’t answer those questions yet and there is a lot of work to be done.”

The United States wants Iran to address questions about its nuclear programme, which Washington and its allies suspect is a cover for developing nuclear weapons. Iran denies that, saying its programme is for peaceful, civilian uses.

Zarif, a US-educated diplomat, also sounded a cautionary note and insisted on quick relief from the painful US, European Union and UN sanctions imposed for Iran’s refusal to suspend its uranium enrichment programme.

“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 told reporters.

“Of course as we move forward, there has to be removal of sanctions and in the end game there has to be a total lifting of all sanctions and both bilateral sanctions, unilateral sanctions as well as multilateral sanctions and UN sanctions and we hope to be able to move in that direction within a short span of time.”

Kerry said Zarif had put some “possibilities” on the table, but stressed there was more work to be done.

A senior US official said after the meeting that Iran had proposed fully implementing an agreement on its nuclear programme within a year. The official added that while Zarif had offered suggestions, “there is a lot more to understand.”

In an interview before Thursday’s meeting, Kerry said the United States would not lift sanctions until Iran showed it was not pursuing a nuclear weapons capability.

He told the CBS show that one concrete step Iran could take to show it was serious about not seeking nuclear arms would be to open up its Fordow uranium enrichment facility to UN inspectors.

“The United States is not going to lift the sanctions until it is clear that a very verifiable, accountable, transparent process is in place, whereby we know exactly what Iran is going be doing with its (nuclear) programme,” he said.

Iran’s new centrist president, Hasaan Rowhani, has said he would like to see a deal with world powers in three to six months.

Kerry told CBS it would be “possible to have a deal sooner than that depending on how forthcoming and clear Iran is prepared to be.”

The meeting was a very uncommon encounter between top officials of the United States and Iran, which have been estranged since the 1979 overthrow of the shah and the taking of American hostages at the US Embassy in Tehran.

A State Department official said it was the highest-level “official” meeting between US and Iranian officials since before the Iranian Islamic Revolution, although there had been “informal” interactions at the same level in the past 10 to 12 years.

Kerry and counterparts from the five permanent members of the UN Security Council and Germany described their meeting with Zarif on the sidelines of the UN General Assembly as “a change in tone” from encounters with Iran’s previous, hard line government.

European Union foreign policy chief Catherine Ashton, who chaired the meeting, told reporters it had been “a substantial meeting. Good atmosphere. Energetic.”

She said the two sides had agreed on an “ambitious timetable” to address Western concerns and would meet again in Geneva on October 15 and 16 “to pursue the agenda to carry on from today’s meeting and to hopefully move this process forward.”

Ashton also added a note of caution, saying it was important to focus on “effective work that we do on the ground.”

The New York talks involved the five permanent members of the UN Security Council – Britain, France, Russia, China, the United States – and Germany, known as the P5+1.

Iran’s new president is a moderate cleric who has stepped up efforts to improve Iran’s image abroad during his visit to New York this week and has said Iran will never develop nuclear weapons.

US President Barack Obama on Tuesday cautiously embraced Rouhani’s gestures as the basis for a possible nuclear deal and challenged him to demonstrate his sincerity.

But the failure to orchestrate a handshake between the two leaders, apparently because of Rowhani’s concerns about a backlash from hardliners at home and perhaps Obama’s concerns about the possibility of a failed overture, underscored how hard it will be to make diplomatic progress.

Even without making any real concessions so far, Rowhani has offered a softer, more reasonable tone than his stridently anti-US predecessor, Mahmoud Ahmadinejad.

The sanctions imposed on Iran have begun to bite severely.

Iranian oil exports have fallen by about 60 per cent in the past two years as the EU stopped purchases and most Asian buyers drastically cut imports because of the sanctions. Iran is now earning only around US$100 million from oil sales a day as opposed to US$250 million two years ago.

The six powers said in February they wanted Iran to stop enrichment of uranium to 20 per cent, ship out some stockpiles and shutter a facility where such enrichment work is done. In return, they offered relief on international sanctions on Iran’s petrochemicals and trade in gold and other precious metals.

US officials say that offer remains on the table.

But signaling some of the obstacles that could hamper any new diplomacy, Iran on Thursday sharply criticised the UN nuclear watchdog over “baseless allegations” about its atomic activity.

It was an apparent reference to the International Atomic Energy Agency’s concerns, spelled out in a series of quarterly reports, about what it calls the possible military dimensions to Iranian nuclear activities.

The Fordow uranium enrichment facility Kerry referred to is a secret plant buried 91 metres underground, making it difficult to bomb should the West or Israel decide they need to take military action against Iran’s nuclear programme.