A top Russian diplomat says work is underway to fulfill Russia's supply of advanced air-defense missiles to Iran, but that no start date for the delivery has been set yet.

Russia in 2010 froze plans to supply S-300 missile systems to Iran, linking the decision to U.N. sanctions. But in April, President Vladimir Putin lifted the suspension 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.

On Tuesday, Russian Deputy Foreign Minister Sergei Ryabkov was quoted by the Interfax news agency as saying he could not "say anything for now" about the missiles' delivery date.