A Kremlin official says that Russia is working to upgrade air defense missiles intended for Iran.

Vladimir Kozhin, a presidential adviser for arms trade, said Thursday according to the RIA Novosti news agency that the S-300 air defense missile systems are to undergo an upgrade before being delivered to Iran, since the original contract was signed a long time ago. He wouldn't offer specifics or say when the delivery would take place.

Russia in 2010 froze a deal to supply S-300 missile systems to Iran, linking the decision to U.N. sanctions.

President Vladimir Putin lifted the suspension in April after a framework Iran nuclear agreement, which earlier this month was followed by the final deal, which curbed Iran's nuclear program in exchange for relief from sanctions stifling its economy.