An Iranian source said Ivanov had been invited by his Iranian counterpart, Ali Larijani, who is also Iran's top nuclear negotiator.

Russia's national security council said that Ivanov's visit would be taking place on Saturday and Sunday.

The trip will follow a meeting in London on Wednesday of senior officials from Britain, France, China, Russia and the United States -- the five permanent United Nations Security Council members -- as well as Germany.

The big powers discussed a European proposal aimed at breaking Iran's determination to enrich uranium, a process which can be extended from making reactor fuel to nuclear weapons.

The foreign ministers of six world powers look set to meet shortly to decide how to proceed on the crisis after progress was apparently made at the London meeting.

Mohamed ElBaradei, director general of the International Atomic Energy Agency, has also suggested that Tehran was willing to compromise on enrichment.

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