Russian Deputy Foreign Minister Sergei Ryabkov says work is under way to fulfill Russia's supply of advanced air-defense missiles to Iran, but that no start date for the delivery has been set yet.
In 2010, Russia froze plans to supply S-300 missile systems to Iran, linking the decision to UN sanctions.
But in April, President Vladimir Putin lifted the ban after a framework agreement was reached that would restrict Iran's capacity to produce nuclear weapons.
The Russian move has been strongly criticized by the United States and Israel.
Ryabkov was quoted by the Interfax news agency as saying on May 26 that he could not "say anything for now" about the missiles' delivery date.
Iranian Deputy Foreign Minister Hossein Amir-Abdollahian said on May 25 that the delivery of Russian S-300 missiles to Iran should take place soon.
"The negotiations on the subject have ended in success. I estimate that the S-300 delivery will take place in quite a short time," he said after meeting his Russian counterpart Mikhail Bogdanov in Moscow. "It will be done at the soonest opportunity possible."