Undersecretary of State Nicholas Burns complained that negotiations at the United Nations over what sanctions to impose on Iran for refusing to freeze its uranium enrichment program had dragged on for three weeks with insufficient progress.

"We have had a lot of debates, a lot of discussions, it's time to get on with it," Burns told reporters.

"We hope very much that the Russian and Chinese governments will accelerate the pace of work in New York," he said.

The major powers have been debating a draft resolution drawn up by Britain, France and Germany that would impose limited sanctions on Iran's nuclear and ballistic missile sectors for Tehran's failure to comply with an earlier UN resolution on halting enrichment.

China and Russia, both close economic partners with Iran, argue the measures are too extensive, while Washington has pressed for tougher action.

US President George W. Bush was due to raise the issue with his Russian and Chinese counterparts, Vladimir Putin and Hu Jintao, over the weekend on the sidelines of a Pacific Rim summit in Hanoi, Burns said.

Despite the slow pace of progress, Burns was optimistic the UN Security Council would eventually impose sanctions on Iran under Chapter 7 of the UN Charter, making the measures mandatory for all UN members.

"These are very complex and difficult negotiations, but we're working hard and I for one believe that we will have a sanctions resolution," he said.

The US and others believe Iran's uranium enrichment program is ultimately aimed at producing fissile material for nuclear weapons.

Iran insists it will use the enriched uranium only to fuel nuclear power stations, something it is permitted to do as a signatory to the nuclear non-proliferation treaty.

Burns said a report issued this week by the Vienna-based International Atomic Energy Agency was "filled with concerns about the lack of transparency" of the Iranian nuclear program.

The report, leaked to reporters, notably said that unexplained traces had been found in Iran of highly enriched uranium and plutonium, materials that can be used in nuclear warheads.

Russian Foreign Minister Sergei Lavrov said earlier Friday that "rather than punishing Iran", the UN Security Council should give the lead role in the crisis to the IAEA, which is tasked with inspecting nuclear sites to ensure fissile material is not used for military purposes.

"The role of the Security Council is precisely to help and not to replace the IAEA in the process," Lavrov told reporters during a visit to India.

"We have been trying to help with the IAEA efforts to bring the matter to the negotiating table rather than block the pathways to the possibility of pursuing those talks," Lavrov said.

Burns described the US-Russian differences as "tactical disagreements" over which sanctions to impose on Iran now and which to hold back for use later in the even Tehran continues to defy the Security Council.

"We don't have a strategic disagreement," he said.

"No one is taking the position that there will not be sanctions, the only question is: What is the framework of this first sanctions resolution and then where do we go from there?", he said.

"I do believe we can work this out."

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