The United States and Russia Thursday clashed over Iran's first nuclear plant which Russia is helping build after strongman Prime Minister Vladimir Putin said the station would come online this summer.

US Secretary of State Hillary Clinton, in Moscow hoping to secure Russia's support for a tougher line on Iran, said such a move would send Tehran the wrong message.

"The launch of the first unit of Iran's nuclear power station should be implemented already this summer," Putin said at a meeting in the city of Volgodonsk in southern Russia in remarks released by his government.

The prime minister is due to meet Clinton on Friday.

Putin is considered the country's top decision-maker and his comment represented one of the clearest recent statements by a Russian official about when the controversial facility will finally come online.

Russia has been helping Iran build the plant in the southern city of Bushehr since the mid-90s but its launch has been marred by a series of delays and the issue remains hugely delicate amid the standoff over Iran's nuclear programme.

Tehran is accused by the West of planning to build a nuclear bomb although it insists its nuclear drive is purely peaceful in nature.

Putin's announcement drew an immediate reaction from Clinton, who has been looking over the past year for Russia to take a tougher line on Iran.

"We have consistently said that Iran is entitled to civil nuclear power," she said at a news conference with her Russian counterpart Sergei Lavrov in Moscow.

"It is a nuclear weapons programme that it is not entitled to and if it reassures the world, or if its behaviour has changed because of international sanctions, then it can pursue peaceful nuclear power."

"In the absence of these 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."

Clinton said the Iranian nuclear programme "remains an issue of grave concern for the international community".

Speaking alongside Clinton, Lavrov insisted however Russia would complete the construction.

"Now appropriate steps to guarantee all the technical requirements are entering their final phase," Lavrov said.

Vladimir Pavlov, an official with Russia's nuclear contractor Atomstroiexport overseeing the construction of the Iranian nuclear plant, said the launch was being planned for July, the Interfax and Ria-Novosti news agencies reported from Volgodonsk.

Trial runs would start in April, when major equipment would be tested, Pavlov said.

Iran's Deputy Foreign Minister Mohammad-Mehdi Akhondzadeh said earlier Thursday Tehran was keen to start the station "as soon as possible."

"We are hoping those responsible in Russia will fulfill their promises on the launch of the nuclear power station in Bushehr," he said in Moscow.

At the meeting in Volgodonsk, Putin tasked officials with boosting Russia's share on the global nuclear market to 25 percent from the current 16 percent, saying Russia was in the process of designing and building 15 nuclear power reactors at home and abroad.

"We have a solid set of orders but we need to move forward," Putin said in televised remarks.

Russia is constructing or gearing up to construct nuclear reactors in a host of countries, including China, India and Bulgaria and Putin said the country was in talks to build more nuclear units in Turkey, Armenia and several other countries he did not identify.

His lightning trip to New Delhi late last week yielded a broad agreement that foresees the construction of up to 16 nuclear reactors at three Indian sites in the coming years, a major triumph for Russia's state atomic agency Rosatom which faces stiff competition in the country from French and US rivals.