Russia on Tuesday confirmed its decision to deliver S-300 air defence missile systems to Iran, but said it could not yet announce a date.
"The decision on delivering S-300 to Iran has been taken but the realisation of the project will take some time," Yevgeny Lukyanov, deputy head of Russia's security council, was quoted by Russian agencies as saying.
"As I understand, the time of delivery has not come yet," he said.
Talks on the controversial deal, which has been frozen since 2010, finished Monday with Iran's Deputy Foreign Minister Hossein Amir-Abdollahian pronouncing them a "success".
Russia's Deputy Foreign Minister Sergei Ryabkov said Tuesday that the agreement is still being finalised.
President Vladimir Putin last month lifted a ban on supplying Iran with the sophisticated systems, following the landmark framework deal Tehran struck with the West over its nuclear programme.
He said several days later that Iran is "demonstrating very high flexibility and clearly wishes to reach a compromise on this nuclear programme," which is why Moscow lifted its own ban.
The decision sparked condemnation from Israel and concern from Washington, as it came before the lifting of the sanctions by the UN Security Council, which originally caused Moscow to freeze the deal.
Now Russia is arguing that the missile system is exclusively defensive and does not even fall under the sanctions.
Iran's Amir-Abdollahian predicted that the delivery would go through "at the soonest opportunity possible".
Iran's deputy oil minister Amirhossein Zamani-Nia said Monday he hoped for a total lifting of international sanctions later this year if world powers resolve all remaining contentious issues and strike the nuclear deal by its June 30 deadline.