Russia has said that it may support a new set of UN sanctions against Iran if it does not stop work that may lead to the creation of nuclear weapons.
Tehran says its nuclear programme is entirely civilian
Western powers suspect Iran of developing weapons, and want it be subjected to sanctions in addition to those imposed in 2006 and 2007.
Correspondents say Russia has until recently been reluctant to impose further sanctions.
Iran denies it has a secret nuclear weapons programme.
Asked by journalists if Russia would support sanctions, Moscow's UN ambassador Vitaly Churkin said: "Yes. If Iran in the next few days does not stop the enrichment activities of its heavy water project then yes, Russia... has taken upon itself certain commitments... to support the resolution that has been drafted in the past month.
"Russia is constantly insisting that the [UN] Security Council adopt certain sanctions against Iran," he added.
Heavy water reactors produce plutonium, which can be an alternative route to a nuclear device, the other being highly-enriched uranium.
Last week, the UN nuclear watchdog said Iran was being more transparent, but had not given "credible assurances" that it was not building a bomb.
The International Atomic Energy Agency (IAEA) said Iran had granted access to sites but remained evasive on key issues.
The UN Security Council imposed two rounds of sanctions in December 2006 and March 2007.
The first prevented the "supply, sale or transfer" of all goods linked to Iran's nuclear work, while the second prevented dealings with the Iranian state bank Sepah and 28 named people and organisations, many connected to the elite Revolutionary Guard. Imports of arms from Iran are banned while loans are supposed to be limited to humanitarian and development purposes.
The third sanctions resolution - formally submitted by France and Britain - calls for asset freezes and mandatory travel bans for specific Iranian officials. It also expands the list of Iranian officials and companies targeted by the sanctions.
Russia's growing ties to Iran's energy industry have made it reluctant to impose more sanctions. Russia is helping to build a nuclear plant in Bushehr, Iran. It has also just finished delivering nuclear fuel to this plant under a $1bn (£501m) contract. Russian gas monopoly Gazprom is also working to develop Iranian gas fields.
Tehran refuses to stop enriching uranium. It says its nuclear work is aimed at generating electricity.