"It is impossible to derive raw material for nuclear weapons from the material that we will be supplying to Iran -- even US experts agree with this," said Viktor Kozlov, who heads the AtomStroiExport company which is building the Bushehr plant.
Iran has dogged Russian-US relations for years and is likely to surface again at next month's Camp David summit between presidents George W. Bush and Vladimir Putin -- just as it did during their last meeting in Saint Petersburg on June 1.
Senior Russian officials accused Washington of actually worrying about competition in the lucrative nuclear power plant construction market rather than about Iran's potential nuclear ambitions.
"The Americans are not concerned about the proliferation of weapons of mass destruction -- they just want to limit Russia's role on the nuclear energy market," said Radzhab Safarov, a presidential adviser on Iranian affairs.
Meanwhile the atomic energy ministry said in a statement that Washington had offered no proof of how the Bushehr project could help Iran build a nuclear bomb.
"In order for Russia to tear up this agreement (with Iran), we need to be presented with firm evidence -- both logistical and political -- and none has come so far," the atomic energy ministry said.
The deal is worth some 800 million dollars (734 million euros) to Russia.
The United States on Wednesday renewed longstanding opposition to Russia's nuclear cooperation with Iran ahead of next month's meeting of the International Atomic Energy Agency (IAEA) that will address the Iranian program.
The State Department said no nation -- including Russia -- should be assisting Iran in its nuclear efforts until Tehran agrees to allow snap inspections of its installations by the IAEA which has raised concerns about the scope of the program.
"Until Iran satisfies the IAEA's questions and fully addresses the concerns of the international community... we believe that no country should be engaging with Iran in nuclear cooperation, and that would include Russia," deputy spokesman Philip Reeker said.
Reeker's comments came one day after the top US arms negotiator left Moscow empty-handed after two days of talks on the issue.
Moscow next month is expected to sign an agreement with Tehran that would ensure that all spent nuclear fuel provided for the Bushehr reactor is returned for reprocessing to Russia.
Russia views this as a key agreement which should allay Western concerns about the possibility of Iran developing nuclear weapons from Bushehr's material.
"I find it strange that the Americans, who for years have been insisting that we sign such an agreement, today are urging us not to hurry and sign the additional protocol," Kozlov of AtomStroiExport said in a Moscow Echo radio interview.
"I do not think that Iran can ever develop a nuclear weapon... because of all of the IAEA controls and because of all the international attention given to the issue," he said.