Western powers have responded with scepticism to a claim by Iran that a deal to swap enriched uranium for nuclear fuel could now be close.
Iran's Foreign Minister Manouchehr Mottaki told a security conference in Germany that an agreement could be reached in a "not too distant future".
But the US and European Union said they were unconvinced and Iran must make a meaningful offer or face new sanctions.
China, which opposes further sanctions, said talks were at a "crucial stage".
The US and its allies fear Iran is attempting to develop nuclear weapons. Iran insists its nuclear programme is peaceful in purpose.
US Defence Secretary Robert Gates, in Ankara, is quoted by Reuters news agency as saying that Western powers needed to think about whether it was now time for a "different tack".
German Foreign Minister Guido Westerwelle told the annual Munich security conference: "Our hand is still reaching out towards them [Iran]. But so far it's reaching out into nothingness.
"And I've seen nothing since yesterday [Friday] that makes me want to change that view."
The US National Security Adviser, General James Jones, warned of tighter sanctions and deeper international isolation for Iran.
And EU foreign affairs chief Catherine Ashton told the conference that Iran must respond to the head of the UN's nuclear watchdog, the IAEA, over its nuclear programme.
"There is a need to restore confidence in the peaceful nature of Iran's programme," she said, according to Reuters.
"This must be done by dialogue. But dialogue takes two, and I'm ready to engage in meaningful and productive talks that deal directly with the issues that trouble us."
The BBC Tehran correspondent Jon Leyne, reporting from London, says the strong suspicion is that the Iranian remarks are just another attempt to fend off new sanctions being proposed by the United States.
Mr Mottaki made his comments on Friday after deciding to join the Munich conference - a major international gathering of security officials - at the last minute.
He said "conducive ground" on a nuclear fuel deal had been reached and that Iran had demonstrated it was serious.
"Under the present conditions that we have reached, I think that we are approaching a final agreement that can be accepted by all parties," he said.
But Mr Mottaki did not mention the key issue of timing and insisted that the quantity of fuel involved should be up to Iran.
In January, diplomats said Iran had informed the International Atomic Energy Agency (IAEA) that it did not accept the terms of a deal agreed in October by Iran, the IAEA and the so-called P5+1 - the US, Russia, China, UK and France plus Germany.
In response, the US, Britain and France have been pressing for more sanctions and earlier this week circulated a discussion paper on further possible measures against the country.
The move came despite recent comments by Iran's President Mahmoud Ahmadinejad indicating that the country would have "no problem" sending much of its low-enriched uranium abroad so it could be processed into fuel - an arrangement envisaged by the October agreement.
Western diplomats reacted warily to Mr Ahmedinejad's comments.
But China's Foreign Minister Yang Jiechi told the Munich conference that the P5+1 should remain patient and keep pursuing a diplomatic solution to the issue.