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.