Gholamreza Ansari refused to name Iran's possible amendments to a Russian proposal to enrich uranium for the Islamic Republic in its territory.
"If I answer this question, the visit of our delegation will be useless," he said.
The ambassador said earlier that Russia's offer needed additional work.
"We believe that if experts mutually remove certain weak points of the project, then it will be a good project," he said.
The ambassador said the Iranian leadership considered the joint-venture proposal a constructive one, and that he hoped the flaws in the plans could be tackled by February 20.
"The Iranian people are hoping [for this], including me," he said.
Russia's uranium enrichment offer has been seen as a compromise to allay international fears that Iran wants to produce nuclear weapons. Talks over Russia's offer, which were originally scheduled for Tuesday, were postponed at Iran's request until February 20.
