Russian President Vladimir Putin has said that Russia would continue to help Iran develop its nuclear programme despite Washington's objections.
Mr Putin was speaking before a visit to the UK
Mr Putin was speaking in an exclusive interview with the BBC One's Breakfast with Frost.
"We won't let the issue of nuclear proliferation be used to stop Russian companies dealing with Iran," he said, adding that Russia, too, had some questions regarding Iran's nuclear energy programme.
Russia is helping Iran to build an $800m nuclear reactor in the south-western port of Bushehr.
The International Atomic Energy Agency (IAEA) - the United Nations nuclear watchdog - has called on Iran to allow stricter inspections of its nuclear facilities.
US President George W Bush urged the world last Thursday to warn Tehran against developing nuclear weapons.
Mr Putin is due to travel to Britain this week in the first state visit by a Russian leader since 1874.
The Russian leader framed the issue of Russia's relations with Iran as an economic one - and said companies from other countries were trying to secure economic advantages in the country.
"We know that some Western European companies closely co-operate with Iran in that sphere and supply it with equipment that is of dual use, to say the least," Mr Putin said.
"We shall develop our relations on nuclear issues - not only with Iran but with other countries too - depending on how open they are to that established and respected international organisation [IAEA] whose experts we all trust," the Russian leader added.
Last Friday, President Putin said he had been assured by his Iranian counterpart, Mohammad Khatami, that Iran had no plans to develop nuclear weapons.
The IAEA has asked Iran to sign an additional protocol to the Nuclear Non-Proliferation Treaty (NPT) "as soon as possible and without conditions".
First nuclear plant comes online by summer 2004
Has signed up to the
Nuclear Weapons Non-Proliferation Treaty
Can now supply its own power stations with nuclear fuel
Mr Putin told the BBC that Iran was prepared "to join all agreements and to place all its nuclear programs under control".
But it was not clear whether he meant the additional protocol to the NPT which would allow for more intensive and short-notice inspections of Iranian nuclear facilities.
IAEA inspection teams are due to return to Iran shortly.
Mr Putin said that Russia's opposition to the war in Iraq had not damaged ties with Britain, saying that his relationship with Prime Minister Tony Blair was open and friendly.
"We believe it is possible to tell each other what we actually think rather than what our diplomats advise us to say," he said.
But he is scheduled to meet Mr Blair for only 30 minutes on his visit, the BBC's Bridget Kendall says.
Mr Putin told the BBC that Russia will insist that some of the multi-million-dollar contracts Russian oil companies signed with Saddam Hussein's Iraq be honoured.
And he said Russia would not be dissuaded from helping Iran with its nuclear programme by US fears that Tehran is trying to develop nuclear weapons.
As in other recent interviews, he refused to be drawn on the question of whether he would run for re-election.
His term ends in less than a year.