Envoys of six major powers huddled behind closed doors at China's UN mission in New York Tuesday for another round of bargaining on new UN sanctions against Iran, but nothing filtered from their deliberations.

Ambassadors from the five veto-wielding members of the UN Security Council -- Britain, China, France, Russia and the United States -- plus Germany have been meeting almost daily since last Wednesday but made clear that there would not comment on the content of their discussions.

A diplomat close to the discussions said however on condition of anonymity that the Russian side made "some rather constructive proposals" in discussions of a US draft outlining sanctions against Tehran.

The source said the Chinese had yet to give their comments on the draft and it was unclear whether they did so at Tuesday's meeting.

The package, already endorsed by Washington's European allies, would include a full arms embargo, a ban on new investments in Iran's energy sector, restrictions on shipping and finance as well sanctions targeting the business interests of the Islamic republic's powerful Revolutionary Guards, sources said.

Diplomats say they expect weeks of hard-nosed bargaining before a text, likely to be toned down to make it palatable to the Chinese and the Russians, can be brought to a vote by the full 15-member Security Council.

The council has already imposed three sets of sanctions on Iran over its refusal to halt its uranium enrichment program, which the West sees as a cover to acquire a nuclear weapons capability.

Tehran however maintains its nuclear program is peaceful and insists that it is entitled to conduct nuclear enrichment under the nuclear Non-Proliferation Treaty (NPT) it has signed.

The issue is expected to figure prominently next month when a NPT review conference is held at UN headquarters.

Meanwhile Turkey, a non-permanent member of the Security Council, on Tuesday offered to help break a deadlock over an atomic fuel deal for Tehran and insisted that diplomacy is the best way to resolve Iran's nuclear crisis.

Turkish Foreign Minister Ahmet Davutoglu said while on a visit to Tehran that his country, which, along with fellow non-permanent council members Brazil and Lebanon, is cool to fresh sanctions against Iran, was ready to act as an intermediary to settle the issue.

Iran's Foreign Minister Manouchehr Mottaki said Sunday that Tehran planned to open talks with all 15 UN Security Council members in a bid to break a deadlock on a nuclear fuel supply deal.

Under the October scheme, Iran would have been expected to ship out 70 percent of its then stocks of low enriched uranium -- enough to make one nuclear bomb -- in return for the supply of fuel by France and Russia.

The impasse is due to Iran's insistence that it only hand over its enriched uranium stocks as the fuel is supplied, and that the exchange take place on its own soil.

On Sunday, Washington expressed interest in reviving the fuel swap scheme.

"At the heart of this was the proposal that Iran would ship out significant amounts of enriched fuel and there would be an exchange for a corresponding amount of fuel suitable for" the Tehran reactor, State Department spokesman Philip Crowley said.

But Crowley also noted that any new plan would have to be amended to take account of the fact that Iran has had seven months since the original offer in which to enrich further stocks of uranium.