The ambassadors held several hours of talks on April 7 aimed at reaching a draft resolution to be considered by the full 15-nation Security Council. China took part in the meeting despite its public objections to sanctions.

China and Russia have so far refused to back the new measures, mainly aimed at Iran's Revolutionary Guards, proposed by the US and some European states.

Iran insists its uranium enrichment programme is for peaceful purposes.

The proposed fourth round of sanctions does not target the country's oil and gas sector.

At a meeting earlier the same day in the Czech Republic with the Russian leader, US President Barack Obama called for a "strong" fourth round of sanctions.

However, Russian President Dmitry Medvedev said after the two leaders signed a landmark nuclear arms reduction treaty in Prague.

But Mr Medvedev said he will only support "smart" sanctions - designed to change Tehran's behaviour, but not to bring down the Iranian government or impose hardship on Iran's people.

In February, the UN's nuclear watchdog, confirmed that Iran had produced 20% enriched uranium.

Tehran says it needs the more highly-enriched uranium for a research reactor producing medical isotopes, but Western powers fear it is heading towards enriching uranium to the 90% required for a weapon.