US Secretary of State Condoleezza Rice has openly clashed with Russia over whether Iran can pursue a nuclear energy programme.
Ms Rice is trying to drum up support for US pressure on Iran
In talks with Russian officials in Moscow, Ms Rice said Iran needed to recognise its obligations under the Non-Proliferation Treaty (NPT).
But her Russian counterpart, Sergei Lavrov, told reporters that Tehran had the right to enrich uranium.
Iran has walked away from talks aimed at halting its plans to enrich uranium.
The US wants to involve Russia in persuading Iran to return to talks, says the BBC's Jonathan Beale, who is travelling with Ms Rice.
He says the unscheduled Moscow visit is part of a concerted US effort to apply international pressure on Tehran.
Iran and Russia have close energy ties and our correspondent says that while President Vladimir Putin has indicated he does not want Iran to have a weapons programme, Moscow has shown little appetite for punishing Tehran.
Ms Rice has now arrived in London for talks with UK Prime Minister Tony Blair.
Ms Rice has urged Iran to resume talks with the EU or risk referral to the United Nations Security Council - a move which may trigger the possibility of sanctions.
But Mr Lavrov said the matter should be dealt with by the International Atomic Energy Agency (IAEA), adding that Iran, as a member of the NPT, had a right to pursue enrichment of uranium.
The US secretary of state told the BBC Iran should reconsider its position before the IAEA decides to refer its case to the UNSC.
Ms Rice said this would happen "at a time of our choosing".
Last month, Russia abstained in a crucial vote at the IAEA, but Ms Rice said this meant Moscow was waiting to see what decision Tehran made.
The IAEA resolution threatens Iran with UNSC referral unless it abandons its plans to process and enrich uranium - the first stages in developing nuclear weapons.
The IAEA will meet again next month, but it is still not clear if there is enough backing for the move.
This week, Tehran said it would welcome the resumption of talks with the Europeans without preconditions.
Tehran insists its nuclear programme is for peaceful purposes only.
On Friday, the US ambassador to the United Nations, John Bolton, accused Iran of spending 18 years of trying to develop nuclear weapons.