Russia and Saudi Arabia are discussing a date for the agreed delivery of S-400 air defense systems to Riyadh, the Saudi ambassador said Tuesday.

"The talks are still ongoing. They are not over yet," Raed bin Khaled Qrimli told reporters in Moscow after he was asked when the deliveries would begin.

The sale was part of accords inked last year during Saudi King Salman’s historic visit to Moscow, the first such trip by a reigning Saudi monarch. Qrimli said in September there were outstanding technical issues.

The S-400 Triumph is Russia's next-generation mobile surface-to-air missile system.

It can carry three different types of missiles capable of destroying a variety of aerial targets at a short-to-extremely-long range, from reconnaissance aircraft to ballistic missiles.