After weeks of conflicting responses, Manochehr Mottaki, Iran's foreign minister, accepted the uranium-for-nuclear fuel exchange proposal last December "in principle."
But in a likely deal-breaker, he spoke of exchanging the material in phases rather than all at once, as called for in the plan.
Hillary Clinton, the US secretary of state, said on Monday that the Obama administration believes that the way to pressure Iran to come clean on its nuclear ambitions is to impose new sanctions aimed at the country's ruling elite.
William Burns, the US under-secretary of state, who also attended Saturday's talks, said before the meeting that Iran would face consequences if it does not comply with the international proposal to limit its uranium enrichment programme.
However, Russia, and China that was only represented by a lower-level diplomat, are two countries who are sceptical of any new sanctions.
Zhang Yesuim, China's UN ambassador, said Beijing opposes new sanctions against Iran.
He said diplomatic efforts to bridge differences over the country's nuclear programme are taking place.
China, which relies on Iran for much of its energy needs, is a veto-wielding member of the security council, along with the US, Russia, Britain and France.
Opposition from Zhang, who took over the rotating presidency of the UN security council in early January, means the council will almost certainly not discuss Iran sanctions in January.
But whether they may be open to future sanctions talks remains to be seen.