(BBC) - Russian Foreign Minister Sergey Lavrov is to go to Iran on Tuesday for a previously unannounced visit.
Mr Lavrov is due to hold talks with President Mahmoud Ahmadinejad at 1930 local time (1600 GMT).
The visit comes amid rising tensions with Western powers over Tehran's nuclear programme.
Iran has dismissed Western allegations that it is developing a nuclear bomb, saying it is seeking only civilian nuclear energy.
The US has not ruled out using force against Iran, but Russian President Vladimir Putin has warned that Moscow will not accept this option.
On Sunday the head of the UN nuclear watchdog said there was no evidence of Iran developing a nuclear bomb.
But France and the US immediately dismissed the findings.
Iran's refusal to stop enriching uranium - a process which can lead to a nuclear bomb - has led the UN Security Council to impose two sets of sanctions, which the US has followed up with unilateral penalties of its own.