UN sanctions against Iran over its nuclear program have been "completely agreed upon," a Russian source said, and Washington said a resolution could be passed as early as Wednesday.

Iran, meanwhile, warned Russia on Tuesday against siding with Tehran's foes in supporting the sanctions.

"The proposal on the sanctions against Iran has been completely agreed upon," said the Russian source close to United Nations Security Council talks, speaking in Moscow on condition of anonymity.

U.S. Defense Secretary Robert Gates said in London that he was confident that the Security Council would pass a resolution as early as Wednesday.

"One of the many benefits of the resolution is that it will provide a legal platform for individual nations to then take additional actions that go well beyond the resolution itself. I believe that a number of nations are prepared to act pretty promptly," Gates added. "I do not think we have lost the opportunity to stop the Iranians from having nuclear weapons."

A Western diplomat at the United Nations said a list of new Iranian individuals and firms to be blacklisted under a fourth round of sanctions had been agreed upon.

Iranian President Mahmoud Ahmadinejad admonished Russia at a news conference in Istanbul, where he was attending a summit along with Prime Minister Vladimir Putin.

"There is no big problem, but they must be careful not to be on the side of the enemies of the Iranian people," Ahmadinejad said.

Putin, who said he expected to meet Ahmadinejad on the sidelines of the Conference on Interaction and Confidence Building Measures in Asia in Istanbul, said the UN resolution had been "practically agreed" but that sanctions should not be "excessive."

"We will have an opportunity to discuss these problems if my Iranian colleague will have such a need," he said.

"I hold the opinion that this resolution should not be excessive, should not put Iran's leadership, the Iranian people in a tricky situation that creates barriers on the way of development of Iran's peaceful nuclear energy," he said.

The Iranian president said a nuclear fuel swap deal agreed by Tehran with Turkey and Brazil was an opportunity that would not be repeated. The deal, which has been rejected by the West as too little too late, was intended to defuse the crisis.