UPDATE 2-Russia, US disagree over Iran atomic plant start-up

* Putin says Russia to start up Bushehr in mid-2010

* Clinton: startup premature without Iran atom assurances

* Opening of the plant comes after years of delays

* Russia so far ambiguous about Iran sanctions

(Updates with Clinton, reactor start-up month, background)

By Darya Korsunskaya

VOLGODONSK, Russia, March 18 (Reuters) - Russia said on Thursday it would start up the reactor it is building at Iran's first atomic power plant in mid-2010, prompting immediate criticism from visiting U.S. Secretary of State Hillary Clinton.

Prime Minister Vladimir Putin announced the startup, but Clinton said such a decision would be "premature" without Iranian assurances on its nuclear programme, which the West fears is aimed at producing atomic weapons.

Russia agreed to build the 1,000 megawatt reactor at Bushehr, on the Persian Gulf, 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.

"We continue work on developing atomic energy capacity both at home and abroad," Putin told a meeting on nuclear energy in the southern Russian city of Volgodonsk.

"The start-up of the first reactor of the Bushehr atomic power station is planned for this summer," he said.

Clinton said this would send the wrong message to Iran, a major oil exporter which faces possible new sanctions over its nuclear programme, which it says is to generate electricity.

Russian Foreign Minister Sergei Lavrov, speaking alongside Clinton after talks in Moscow, responded by saying the Bushehr plant was key to maintaining the presence of the U.N.'s nuclear agency in Iran.

Vladimir Pavlov, the official in charge of the project at state-run Atomostroiexport, the state-controlled company building the reactor, said "the physical launch of the station is scheduled for July."

Clinton was in Moscow for a meeting of Middle East peace mediators and talks with Russian officials including President Dmitry Medvedev.

NUCLEAR FUEL

Iran has received nuclear fuel for the plant from Russia and will have to return all spent fuel rods to Russia -- an arrangement that had eased U.S. concerns that Iran could use it for weapons.

Russia says the plant is purely civilian and cannot be used for any weapons programme as it will come under International Atomic Energy Agency supervision. Iran will have to return all spent fuel rods to Russia.

Russia agreed to build the plant in 1995 on the site of an earlier project begun in the 1970s by German firm Siemens. The Siemens' project was disrupted by Iran's 1979 Islamic revolution and the 1980-88 Iran-Iraq war.

Russia, a veto-wielding permanent member of the United Nations Security Council, has repeatedly delayed the plant amid international efforts to force the Islamic Republic to allay fears over its nuclear programme.

Russia has even been censured by some in Tehran for using Iran and the Bushehr plant as a "pawns" in the diplomatic poker game over Iran's nuclear ambitions.

But officials in Moscow are this year much more upbeat about starting up the reactor at the plant.

Iran's nuclear energy chief, Akbar Saleh, said earlier this month that the Bushehr plant would be launched at the end of spring.

Russia is keen to complete the plant so that it gains authority in the Middle East as a major supplier of nuclear power plants and some analysts say Moscow could bid to build more atomic power stations in Iran.

Moscow has previously supported UN sanctions against Iran only after insisting they be watered down and has so far refused publicly to support calls by the United States for the threat of additional sanctions against the Islamic Republic.

Putin, who many diplomats say takes the lead on formulating Russia's policy towards Iran, rarely comments on Iran though he last year warned the international community against intimidating Iran. (Reporting by Darya Korsunskaya, writing by Guy Faulconbridge, editing by Steve Gutterman and Charles Dick)