The European Union has circulated a US-backed draft resolution calling on the International Atomic Energy Agency (IAEA) governing board to report Iran's secretive nuclear programme to the Security Council, which could impose sanctions.
Western countries suspect Tehran is developing atomic weapons under cover of a civilian nuclear energy programme.
Iran insists its programme is peaceful and intended to meet its energy requirements.
Both Russia and China, which as permanent, veto-wielding members of the council could block any action, warned the West against antagonising Iran.
"While Iran is cooperating with the IAEA, while it is not enriching uranium and observing a moratorium, while IAEA inspectors are working in the country, it would be counter-productive to report this question to the UN Security Council," Russian Foreign Minister Sergei Lavrov said.
"It will lead to an unnecessary politicising of the situation. Iran is not violating its obligations and its actions do not threaten the non-proliferation regime," he said.
Chinese Foreign Minister Li Zhaoxing told an EU team headed by British Foreign Secretary Jack Straw at the United Nations that sending the Iran issue to the Security Council could be counter-productive, a European participant said.
The diplomat quoted Li as saying that kicking the issue from Vienna to New York "could encourage Iran to take extreme measures" and would not be constructive.
Russia is building a $1 billion nuclear reactor for Iran and sees it as a key ally in the Middle East.
"The Russians are blocking the resolution," said a diplomat from one of the EU "big three" countries - France, Britain and Germany - on condition of anonymity.
"They aren't moving at all, not one centimetre. They don't even want to talk about the resolution," the diplomat said.
Western countries say that since Iran hid its uranium enrichment programme from the IAEA for 18 years, the only way it can prove it is not seeking nuclear bombs is permanently to renounce sensitive nuclear technology.
Echoing US language on Iran, the EU hardened its rhetoric, blasting Iran for its determination to press ahead with a programme which could produce atomic bombs.