Iran says its nuclear programme is entirely peaceful
The UN ambassadors of six world powers have met in New York to discuss possible new sanctions against Iran over its nuclear programme.
The envoys described the talks as worthwhile, and said meetings would continue in the coming weeks.
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.
The ambassadors held several hours of talks 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.
Afterwards, its ambassador said the talks were part of a diplomatic process, and would not be drawn on whether Beijing would support tougher measures against Iran.
At a meeting earlier on Thursday in the Czech Republic with the Russian leader, US President Barack Obama called for a "strong" fourth round of sanctions.
Speaking at Mr Obama's side, Russian President Dmitry Medvedev gave Moscow's strongest support yet for considering a fourth round of UN sanctions.
"Unfortunately Tehran is not reacting to an array of constructive compromise proposals," Mr Medvedev said after the two leaders signed a landmark nuclear arms reduction treaty in Prague.
But Mr Medvedev said he would 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.
Asked to clarify what "smart" sanctions entailed, Russian Deputy Foreign Minister Sergei Ryabkov said Moscow would not, for example, endorse a total embargo on the delivery of refined petroleum products to Iran.
He said such products might be targeted in other ways, or sanctions on Iran's energy sector might have to be avoided altogether.
Russia, like China, has the power to veto sanctions.
The talks came while Iran's top nuclear official, Saeed Jalili, was on a visit to China.
Beijing has a close diplomatic and trade relationship with Iran, dominated by its imports of Iranian energy resources.
Iran's Saeed Jalili has dismissed sanctions as ineffective
"China still believes dialogue and consultation are the best way to solve the nuclear issue," foreign ministry spokeswoman Jiang Yu told reporters on Thursday.
UN diplomats said a US draft proposal agreed with European allies and passed on to Russia and China a month ago had formed the basis of discussions.
The US Ambassador to the UN, Susan Rice, told reporters ahead of the talks that the parties were working to reach an agreement "within a matter of weeks".
There have been three rounds of UN sanctions against Iran, blocking trade of "sensitive nuclear material", freezing the financial assets of those involved in Iran's nuclear activities, banning all of Iran's arms exports and encouraging scrutiny of the dealings of Iranian banks.
The US and its allies on the UN Security Council have been pushing for a fourth round, which would target Iran's powerful Revolutionary Guard, which has major interests in nuclear proliferation activities.
The sanctions would also toughen existing measures against Iran's shipping, banking and insurance sectors, and target additional companies and individuals connected to its nuclear programme, diplomats familiar with the draft said.
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%.
A nuclear weapon normally contains uranium enriched to approximately 90% or more, although a crude device could be constructed using 20% enrichment.