Russia has urged Iran to co-operate with a demand from the UN's nuclear agency to prove that it is not secretly developing atomic weapons.
Tehran denies it has a nuclear weapons programme
Iranian delegates walked out of a meeting of the International Atomic Energy Agency (IAEA) on Friday after a resolution was adopted giving Iran until 31 October to disclose all its nuclear activities.
"It is in Iran's interest to clear these issues and thus confirm the peaceful nature of its nuclear programmes," Russian Deputy Foreign Minister Sergei Kislyak said.
Russia is building Iran's first nuclear power reactor at the port of Bushehr.
Correspondents say Moscow has come under intense pressure from the US to drop the project because of fears that Tehran may use spent fuel from the plant to develop nuclear weapons.
For its part, Washington has warned that Iran's failure to work with the IAEA programme will constitute proof that it is pursuing a secret weapons programme.
Mr Kisylak told the Russian Interfax news agency that Iran should not see the IAEA deadline as an ultimatum.
"It is a serious and respectful appeal by the agency for Iran to co-operate with the IAEA with a view to lifting all remaining questions, and do so without delay," he said.
But the IAEA decision provoked a furious reaction from the Iranian Government.
Iran's ambassador to the IAEA, Ali Akbar Salehi, said the resolution showed that the US intended to invade Iran as it did Iraq and said they would begin a "deep review" of their relations with the nuclear watchdog.
"It is no secret that the current US administration, or at least its influential circle, entertains the idea of invasion of yet another territory, as they aim to re-engineer and re-shape the entire Middle East region."
Friday's resolution does not outline consequences of Iranian non-compliance, but leaves open the possibility of UN Security Council involvement.
A spokesman for the US state department said: "If Iran fails to take those steps by the deadline, that
would constitute further evidence of its ongoing efforts to conceal its clandestine activities.
"Unless Iran immediately reverses course, [the IAEA would be obliged] to report Iran's non-compliance to the UN Security Council," Adam Ereli said.
IAEA Director General Mohamed ElBaradei said the agency would adopt "a very vigorous approach to complete our work and we will make sure we get all the information we need".
Iran has some serious decisions to take and time is running out, says the BBC's Tehran correspondent Jim Muir.
It can suspend co-operation with the IAEA and adopt a stance similar to North Korea's - as some hardliners advocate.
Or it can open up fully to the inspectors, to prove it is not building nuclear weapons, our correspondent says.
The United States has accused Iran of covertly developing nuclear weapons under the guise of a nuclear energy programme.
But Iran says it is only seeking to produce low-grade uranium fuel to meet its energy needs.
The 35-nation governing board of the IAEA met all week, examining a report by IAEA inspectors, which says traces of weapons-grade uranium were found at an Iranian nuclear plant.
The IAEA will decide the next step if it finds Iran has not co-operated when it meets again in November.