Russia's envoy to Tehran on Sunday urged Iran to sign a UN-drafted nuclear fuel deal to help resolve the controversy over its atomic drive, which he said lacks "complete transparency."

"This is not to trick Iran in order to take its low-enriched uranium out of its hands," Moscow's ambassador to Tehran Alexander Sadovnikov said in an interview with the official IRNA news agency.

"We believe that reaching this agreement and signing the technical contract to produce fuel for the Tehran reactor is beneficial to Iran and will help in resolving the nuclear issue," he said.

The UN-brokered deal has been facing stiff opposition from top Iranian officials who say it is a Western sleight of hand aimed at getting the Islamic republic to suspend its uranium enrichment work.

Sadovnikov's comments come a day after US Secretary of State Hillary Clinton warned Tehran that the patience of world powers had limits as Iran dilly-dallied over offering a categorical response to the nuclear fuel deal.

Sadovnikov said fresh international sanctions against Tehran were no answer to halt Iran's galloping nuclear drive, pursued despite three sets of UN sanctions.

"We agree with the opinion of our partners in the group of six who believe that Iran's nuclear programme, especially in some issues related to its past nuclear activities, lacks complete transparency," Sadovnikov said.

Russia, and five other world powers -- Britain, France, China, Germany and the United States -- are engaged in high-profile talks with Tehran to allay concerns over its atomic programme.

Western powers led by Washington suspect Iran's nuclear programme, particularly its uranium enrichment drive, is aimed at making weapons, a charge strongly denied by Tehran.

In a bid to resolve the stand-off, Washington is backing the UN-drafted deal which sees Iran's low-enriched uranium (LEU) being sent to Moscow for higher processing and conversion into fuel for a research reactor in Tehran.

Western powers fear Tehran could enrich this LEU to very high levels and use it in making an atomic bomb, and are backing the deal as the fuel which will be returned to Iran will be used in an internationally-supervised facility.

US President Barack Obama, angry after Tehran recently disclosed it was building a second uranium enrichment facility, has indicated that if Iran does not come clean on its nuclear programme it could face a new round of sanctions.

But Sadovnikov said threats of sanctions would not work.

"Russia believes that negotiations must be approached from a calculated position without threats and scare," he said.

"As Russian officials have repeatedly said, threats and sanctions will only complicate the situation and lead to a dead end."

Moscow has recently, however, said that sometimes sanctions are "inevitable."

Sadovnikov insisted Tehran must solve the nuclear crisis but added that Iran's nuclear programme if it stays "peaceful" was not a security threat.

"It is totally appropriate to solve Iran's nuclear issue as quickly as possible... close the case and start international cooperation in all dimensions with Tehran in the nuclear field," he said.

"This cannot be done without arriving at similar views and without making inevitable compromises. Therefore, we look at the next round of talks with hope and expect all the negotiators to be politically wise, constructive and patient."

The six world powers and Tehran are expected to meet in few weeks, after the first round of talk held in Geneva on October 1.

The Geneva talks were followed by a specific round of discussion in Vienna between Russia, France and the United States, where the fuel deal was drafted by UN atomic watchdog, the International Atomic Energy Agency.

Iran has asked for more talks on the fuel deal, while its Foreign Minister Manouchehr Mottaki said in Kuala Lumpur on Sunday that technical experts of world powers too felt a need for a "new round of talks" on issues raised by Tehran.

Israel -- Middle East's sole if undeclared nuclear state -- has said the fuel deal is a "positive step" in stopping Tehran's nuclear drive.