By Arshad Mohammed and Conor Sweeney
MOSCOW (Reuters) - Visiting U.S. Secretary of State Hillary Clinton on Thursday criticized Russia's plans to start up a nuclear power station in Iran, describing them as premature given uncertainty about Tehran's nuclear ambitions.
As Clinton was entering a meeting in Moscow with Foreign Minister Sergei Lavrov, Prime Minister Vladimir Putin announced in a provincial city that Russia would start up the nuclear reactor it is building at Iran's Bushehr plant in the summer.
Asked at a subsequent news conference about Putin's announcement, a stern-looking Clinton responded that if Iran reassured the world that it was not pursuing nuclear weapons, then it could pursue civil nuclear power.
"In the absence of those reassurances, we think it would be premature to go forward with any project at this time because we want to send an unequivocal message to the Iranians," she said.
The United States is leading a drive to impose a fourth round of United Nations Security Council sanctions on Iran to persuade it to abandon its suspected pursuit of nuclear weapons. Tehran denies any such intention.
Lavrov defended Russia's nuclear cooperation with Iran, whose program is monitored by the U.N.'s International Atomic Energy Agency (IAEA).
"Bushehr plays a special role in maintaining the IAEA's presence in Iran, in ensuring that Iran is complying with its non-proliferation obligations," Lavrov said.
Russia agreed to build the 1,000-megawatt reactor at Bushehr 15 years ago but delays have haunted the $1 billion project and diplomats say Moscow has used it as a lever in relations with Tehran. It will be Iran's first nuclear power plant.
The disagreement overshadowed the major goals of Clinton's visit -- to seek Moscow's backing for tougher sanctions against Iran and to clear obstacles to a U.S.-Russian agreement cutting both sides' nuclear arsenals.
Clinton is keen to produce results from the drive to improve relations with Russia which began when she presented Lavrov in March 2009 with a red button labeled "Reset," symbolizing hopes of a fresh start.
Clinton said both sides expected to sign the nuclear arms pact soon, but conceded that negotiators had still not quite finished their work on a new treaty.
"We have a saying in the United States -- don't count your chickens until they hatch," she told reporters.
"And that means that we are beginning our discussions about where and when our two presidents will sign the START agreement but we don't want to get ahead of ourselves. First our negotiators have to sign on the dotted line."
Talks have dragged on in Geneva for almost a year on a new treaty to replace the Cold War-era START I pact, missing an original deadline of December.
Despite numerous assurances from both sides that agreement is near, a final deal has remained elusive.
Raising hopes of a possible breakthrough, U.S. officials said Clinton would meet Putin on Friday, a last-minute addition to her visit.
Diplomats say no significant breakthrough on an arms control treaty or on sanctions against Iran is likely without the agreement of Putin, Russia's most powerful politician.
U.N. Secretary-General Ban Ki-moon, also in Moscow, urged Russia and the United States to sign the new arms control pact "as soon as possible".
Both Clinton and Lavrov praised the more positive climate in bilateral relations but had only one formal announcement to make -- that Washington would share financial intelligence with Moscow on the flow of drugs into Russia.
Clinton and Ban will attend a meeting on Friday of the Middle East peacemaking quartet -- the European Union, Russia, the United Nations and the United States.
The quartet's discussions were designed to show support for indirect talks between Israel and the Palestinians that the United States announced last week. But these were thrown into doubt after Israel unveiled plans to build 1,600 homes for Jews in a part of the occupied West Bank it annexed to Jerusalem.
Clinton has described the announcement -- made while U.S. Vice President Joe Biden was in Israel last week -- as insulting. The State Department said she had yet to speak to Israeli Prime Minister Benjamin Netanyahu about the issue since she presented him with a series of demands last Friday.
A U.S. official who spoke on condition that he not be named said Clinton's demands included that Netanyahu get some measure of control over the progress of such housing developments so that they do not undermine talks.
The official said Washington also wants Israel to deal with the core issues of the six-decade conflict -- which include borders, Jerusalem, the fate of Palestinian refugees and Jewish settlements on the West Bank -- during indirect talks, instead of waiting to address them in direct negotiations.
(Additional reporting by Steve Gutterman and Dmitry Sergeyev; editing by Guy Faulconbridge and Mark Trevelyan)