Russia and Iran signed a memorandum of understanding to deliver S-300 missile defence systems, a senior official in the Russian Federal Service for Military-Technical Cooperation told RIA Novosti on Thursday.
According to Russia's state news agency, the source said the memorandum did not determine the number of systems that Iran is expected to get or the time framework of the deal.
The deal, worth $800 million, was first signed in 2007. However, Russia suspended its implementation after the UN Security Council imposed an arms embargo on Iran due to its nuclear programme.
Last April, Russian President Vladimir Putin announced lifting a ban on providing Iran with the sophisticated system after the latter reached a framework deal with world powers pertaining to its nuclear activities.
The United States expressed objections to Russian-Iranian cooperation in that aspect.
"We've been making very clear our objections to any sale of this missile system to Iran for quite some time, and we'll continue to monitor it closely," CNN quoted US State Department spokesman John Kerry as saying last week.
Iran and the so-called P5+1—five permanent members of the UN Security Council plus Germany—reached a nuclear agreement on July over the nuclear crisis.
The deal, which was reached after a decade of diplomatic efforts, seeks to stop Iran from having a nuclear weapon in return of relief of economic sanctions.