A top Iranian negotiator on Tuesday said "constructive" nuclear talks in Moscow offered hope of ending a diplomatic standoff, but some Russians voiced concern that Iran was using the talks to stall for time and avert international sanctions.

The talks centered on a Kremlin compromise proposal, backed by the United States and the European Union, that calls for uranium for Iran's nuclear energy program to be enriched in Russia to prevent it from being diverted for atomic weapons.

Enriched uranium can be used as fuel for a nuclear reactor or fissile material for a bomb. Iran says it is pursuing peaceful nuclear energy but Western nations fear it is seeking an atomic weapon.

"In our belief, the trend of negotiations was positive and constructive," Iran's chief negotiator, Ali Hosseinitash, told state-run Iranian television as he left Moscow after two days of meetings.

"There are elements in these negotiations that give us grounds for hope that we will reach an agreement," Russia's Interfax news agency quoted Hosseinitash, the deputy secretary of the Supreme National Security Council, as saying.

The negotiations ended without any visible progress, but Russian Foreign Minister Sergey Lavrov declined to label them a failure.

"I would be cautious about using the term 'failure' or 'setback' as long as the negotiations have not finished," he said in televised remarks.

U.S. Undersecretary of State Nicholas Burns said Tuesday on a visit to Moscow that "from what we know ... no new ground was broken," but added that he thought broad cooperation among countries in pressuring Iran was working well.

"I think the momentum is with this coalition," Burns told reporters.

Russia's atomic chief, Sergei Kiriyenko, who is set to visit Iran Thursday for further talks, said that, "Russia will do everything possible to offer Iran an opportunity to get out of this difficult situation in a peaceful, constructive way."

But a senior Russian lawmaker expressed frustration.

"Unfortunately, Iran so far has not shown sufficient good will," Konstantin Kosachev, head of the lower house of parliament's foreign affairs committee, was quoted as saying by news agencies.

Russia's offer is seen as the final opportunity to ease international concerns over Iran's suspected nuclear weapons drive. Iran has insisted on its right to maintain domestic enrichment despite international calls for it to stop.

The U.N nuclear watchdog, the International Atomic Energy Agency, is to hold a March 6 meeting that could start a process leading to punishment by the U.N. Security Council, which has the power to impose sanctions on Iran.

Russia, which has strong economic ties to Iran, is building the theocracy's first nuclear power station and is anxious to avoid sanctions and eager to win prestige by helping find a solution.

China, which like Russia has resisted strong measures against Iran, joined calls on Tehran to freeze enrichment.

"We hope Iran can restore its moratorium on all activities related to uranium enrichment and create the conditions for the solution of the nuclear issue through negotiations," Chinese Foreign Ministry spokesman Liu Jianchao said Tuesday.

Sergei Markov, a Kremlin-connected political analyst, said Tuesday that Iran appeared to be deliberately stringing Russia along in the hope of avoiding Security Council action.

"Iran will seek to drag out the negotiations, because while they are ongoing, the possibility of referral to the U.N Security Council and the possible implementation of economic sanctions are almost zero," he was quoted as saying by Interfax.