The UN secretary-general told Reuters that Mohamed ElBaradei, the director-general of the Vienna-based International Atomic Energy Agency, did not expect to have a report on Iran ready until the end of February, for the IAEA's regular board meeting in March.
"I am not sure that they will be ready to refer [Iran] to the Security Council if the official report of the board has not been released and deliberated upon by the board and a decision taken," Annan said on the sidelines of the World Economic Forum.
The IAEA is scheduled to discuss the situation on 2 February.
The United States and the European Union say that it is time for the IAEA to turn Iran's nuclear dossier over to the Security Council. Russia and China are urging caution.
The United States, Britain, France, Russia and China - the five veto-wielding powers of the 15-member Security Council -plus Germany plan to meet in London on Monday to try to resolve differences over whether to send Iran to the council.
Ali Larijani is keen on Russian
co-operation to purify uranium
Russia and China have so far opposed a formal referral.
Annan said he was encouraged by comments from Ali Larijani, Iran's senior nuclear negotiator, who said after talks in Moscow that he was positive about the idea of setting up a joint venture with Russia to purify uranium on Russian soil.
Larijani said the plan could be "perfected" during talks in Moscow scheduled for 16 February - two weeks after the IAEA board meeting.
Annan previously appealed to Iran to scrap its decision to stop nuclear research and return to negotiations with France, Britain and Germany.
He has said a Security Council referral, which could lead to sanctions, should be a last resort.
"It is also encouraging to hear the Iranians say they are considering the Russian offer very seriously. It is a solution that the international community is ready to accept"
On Wednesday, he said: "As you know, I have appealed to the Iranians not to escalate, maintain the situation as it is today in order to create the right environment and confidence that negotiations are serious and lead to concrete results that would be mutually acceptable.
"It is also encouraging to hear the Iranians say they are considering the Russian offer very seriously. It is a solution that the international community is ready to accept."