PARIS, March 19 — Russia has informed Iran that it will withhold nuclear fuel for Iran’s nearly completed Bushehr power plant unless Iran suspends its uranium enrichment as demanded by the United Nations Security Council, European, American and Iranian officials say.

The ultimatum was delivered in Moscow last week by Igor S. Ivanov, the secretary of the Russian National Security Council, to Ali Hosseini Tash, Iran’s deputy chief nuclear negotiator, said the officials, who spoke on condition of anonymity because a confidential diplomatic exchange between two governments was involved.

For years, President Bush has been pressing President Vladimir V. Putin of Russia to cut off help to Iran on the nuclear power plant that Russia is building at Bushehr, in southern Iran. But Mr. Putin has resisted. The project is Tehran’s first serious effort to produce nuclear energy and has been very profitable for Russia.

Recently, however, Moscow and Tehran have been engaged in a public argument about whether Iran has paid its bills, which may explain Russia’s apparent shift. But the ultimatum may also reflect an increasing displeasure and frustration on Moscow’s part with Iran over its refusal to stop enriching uranium at its vast facility at Natanz.

“We’re not sure what mix of commercial and political motives are at play here,” one senior Bush administration official said in Washington. “But clearly the Russians and the Iranians are getting on each other’s nerves — and that’s not all bad.”

A senior European official said: “We consider this a very important decision by the Russians. It shows that our disagreements with the Russians about the dangers of Iran’s nuclear program are tactical. Fundamentally, the Russians don’t want a nuclear Iran.”

At a time of growing tensions between Washington and Moscow, American officials are welcoming Russian support on the situation with Iran as a sign that there are still areas in which the two powers can cooperate.

Russia has been deeply reluctant to ratchet up sanctions against Iran in the Security Council, which is expected to vote on a new set of penalties against the country within the next week.

But American officials have been trying to create a commercial incentive for Russia to put pressure on Iran. One proposal the Bush administration has endorsed since late 2005 envisions having the Russians enrich Iran’s uranium in Russia. That creates the prospect of tens or hundreds of millions of dollars in business for Russia, and a way to ensure that Iran receives only uranium enriched for use in power reactors, instead of for use in weapons.

Iran has rejected those proposals, saying it has the right to enrich uranium on its own territory.

The Russian Atomic Energy Agency, or Rosatom, is eager to become a major player in the global nuclear energy market. As Security Council action against Iran has gained momentum and Iran’s isolation increases, involvement with the Bushehr project may detract from Rosatom’s reputation.

In a flurry of public comments in the past month, Russian officials acknowledged that Russia was delaying the delivery of fuel to the reactor in the Iranian port city of Bushehr. It blamed the decision on the failure of Iran to pay what it owes on the project, not on concerns about nuclear proliferation.

But last month, Foreign Minister Sergey V. Lavrov informed some European officials that Russia had made a political decision not to deliver the fuel, adding that Russia would state publicly that the sole reason was financial, European officials said.

And then last week, a senior Iranian official confirmed in an interview that Mr. Ivanov had threatened Iran with an ultimatum: The fuel would be delivered only after Iran’s enrichment of uranium at Natanz was frozen.

Members of the Security Council are moving toward a vote this week on a draft resolution imposing further sanctions on Iran for its defiance of demands that it suspend enrichment activities and return to negotiations over its nuclear program.

The resolution focuses on the country’s arms exports, a leading Iranian bank and the elite Revolutionary Guards military force. It will reduce Iran’s access to foreign currency and isolate the bank, Bank Sepah, from international financing.

The United States State Department has granted visas to President Mahmoud Ahmadinejad of Iran and a retinue of 38 aides and security staff so that he can address the Security Council meeting.

Throughout the negotiations, the Russians tried to water down the resolution, a reflection of both their desire to avoid a backlash in Iran and their strong skepticism about the effectiveness of sanctions.

The pending resolution follows a similar one passed in December that required four months of negotiations, in large part because of Russia’s resistance. Russia’s support came only after an initial proposal, which would have imposed curbs on Bushehr, was dropped.

Russian officials have gone out of their way to not publicly link the Bushehr project and the crisis over Iran’s decision to forge ahead with producing enriched uranium, which, depending on the level of enrichment, can be used to produce electricity or make weapons.

In remarks on Sunday, for example, Mr. Ivanov said there should be no linkage between discussions on Iran’s nuclear program and the Bushehr plant. “It is a separate issue,” he told a conference of Russia’s Foreign and Defense Policies Council. He added, “All the work being done is under strict control of the International Atomic Energy Agency,” the United Nations’ nuclear watchdog agency based in Vienna.

He also cautioned against using possible nuclear sanctions for other purposes, saying, “We oppose attempts to use this issue as an instrument of pressure or interference in Iran’s internal affairs.”

But Mr. Ivanov also called on Iran to resolve outstanding questions with the agency about its nuclear program and to stop enriching uranium. The Russians have been pressing Iran to take some sort of pause in its uranium enrichment that might allow the Security Council sanction process to halt and bring Iran back to the negotiating table.

“The clock must be stopped; Iran must freeze uranium enrichment,” Mr. Ivanov said. “The U.N. Security Council will then take a break, too, and the parties would gather at the negotiating table.”

The head of the International Atomic Energy Agency, Mohamed ElBaradei, has also called for a “pause,” noting that even a brief suspension of enrichment would be enough to get the United States to the negotiating table with Iran under an offer that Secretary of State Condoleezza Rice made in May.

The Bushehr nuclear project has a long history. For more than a decade, Russia has been working under a $1 billion contract to complete the plant, which began with Germany during the time of Shah Mohammed Reza Pahlavi. After the 1979 revolution, the project was halted; then the site was bombed by Iraq during its eight-year war with Iran. When Iran decided to complete the facility after the war ended, Germany, under pressure from the United States, refused to finish it, or even provide Moscow with the original blueprints.

The project — already eight years behind schedule — is now almost complete. Last year, Russia agreed to ship low-enriched fuel to the plant by March 2007 and start it in September, with electricity generation to start by November.

But in mid-February, Russia said Iran had not made the last two $25 million monthly payments after insisting that it be allowed to pay in euros instead of dollars. Russian officials cited a delay in the delivery of safety equipment from an unspecified third country as another reason for the decision.

Iranian officials denied that payments had been delayed. “Iran has had no delay whatsoever in making payments for the Bushehr nuclear power plant,” Mohammad Saeedi, deputy head of the Atomic Energy Organization of Iran, was quoted by Iran’s state-run news agency IRNA as saying after the Russian claim.

“We would be crazy at this late date to endanger the project by not paying,” the official said. “There is no financial problem. The Russians want to use this issue as a bargaining chip.”