The United States and other world powers are urging Iran to cooperate with the International Atomic Energy Agency and freeze all work on uranium enrichment.
The European Union and Russia joined Washington in saying Tehran should abide by an IAEA resolution adopted Saturday.
Russia's Foreign Ministry says Moscow backs the resolution as a way for Iran to answer questions about the aim of its nuclear program.
Russia is helping Iran to build a nuclear reactor, over objections from the United States, which has accused Iran of striving after nuclear weapons.
Iran denies that charge but on Sunday, it rejected the IAEA's demands as illegal under the nuclear Non-Proliferation Treaty.
Monday, an Iranian spokesman said Tehran's earlier decision to suspend uranium enrichment was voluntary, and the program could be resumed at any time.
Information for this report is provided by AP, AFP and Reuters.