Russia is expected to oppose a US-led push for sanctions after Iran refused to stop enriching uranium by August 31 as demanded by the United Nations Security Council.
China has also said that it favours further attempts to negotiate with Iran and said sanctions may be "counter-productive".
Javier Solana, the European Union foreign policy chief, is expected to meet Ali Larijani, Iran's chief nuclear negotiator, in Vienna on Wednesday to explore the suggestion that Iran is willing to discuss the programme.
Diplomats say there is little enthusiasm for sanctions in the 25-nation bloc as many believe there is still the opportunity for negotiation.
Iran, the world's fourth largest oil exporter, says its enrichment of uranium is legal and designed to meet civilian energy needs. The US fears that it is a front for developing the technology to produce nuclear weapons.
Gregory Schulte, the US ambassador to the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), said on Tuesday: "That's our judgement. Now, there's not 100 per cent certainty there. We've been wrong in the past with our judgements.
"Sanctions are not the end of diplomacy. [But] ... to be successful, diplomacy must now be backed up by sanctions, applied in a graduated fashion and targeting Iran's weapons programme and those who guide it."
George Bush, the US president, has pledged to work closely with other governments to find a diplomatic solution to the stand-off but said: "The world's free nations will not allow Iran to develop a nuclear weapon."
Igor Shuvalov, a senior aide to Vladimir Putin, the Russian president, has not said how Russia might vote at the Security Council but he defended Moscow's opposition to sanctions.
He said: "We are for using a stick [against Iran] but we do not want to later get hit on the head by that same stick, or that it should hit any of our partners on the head.
"At the moment we have understood that introducing sanctions now or resorting to a military operation could lead to the ... consolidation of the population around the current leadership."
Shuvalov did not rule out Russian support for sanctions at some point but he said the international community must "show particular caution and responsibility".
Russia has a contract with Tehran to build a nuclear power station at Bushehr and Russian oil companies have interests in the country.