U.S. Secretary of State Hillary Rodham Clinton urged Russia Thursday to delay launching Iran's nuclear plant until Tehran proves that it's not pursuing atomic weapons.

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 e1 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.

Clinton said Iran is entitled to civil atomic energy, but added that it would be premature to go forward with any nuclear project when Tehran has yet to prove the peaceful nature of its program.

‫"We discussed at length Iran?s nuclear program, which remains an issue of grave concern for the international community," Clinton said, adding that while the two superpowers were "still committed, as we have been, to a diplomatic solution, but there must be a solution."

Hinting at the possibility of a new round of sanctions against Iran's nuclear program, Secretary of State Clinton said that "Iran is not living up to its international obligations and, therefore, we?re working together with our other partners in the P-5+1 to bring together a very clear international consensus in the Security Council that gives Iran the message it needs to hear that its behavior does have consequences and that its pursuit of nuclear weapons poses a direct threat both to regional and global security.‬"

The U.S. and other nations are concerned that Iran has been trying to secretly develop nuclear weapons.

Russian Foreign Minister Sergey Lavrov responded that Russia still intends to launch the plant in the Iranian port city of Bushehr. Both spoke after their talks in Moscow.

Earlier President Barack Obama said the United States would pursue "aggressive sanctions" to prevent Iran from getting a nuclear weapon that could potentially spark a nuclear arms race in the Middle East.

Obama, who had made the goal of pursuing dialogue with Iran a cornerstone of his administration's foreign policy at the beginning of his presidency, said he had been successful in getting the international community to isolate Tehran.

Iran denies it is seeking to build a nuclear bomb and says its nuclear program is aimed at generating electricity.

Obama said preventing Iran from acquiring a nuclear weapon was one of his administration's highest priorities.

"It is a hard problem but is a problem that we need to solve because if Iran gets a nuclear weapon then you could potentially see a nuclear arms race throughout the Middle East and that would be tremendously damaging to our national security interests," he said.

U.S. officials said on Tuesday the pace of Iran's nuclear weapons development appears to have slowed, buying time for a new round of sanctions now and potentially more sweeping measures later.