Saudi Arabia says negotiations over the purchase of Russian S-400 missile system are “advancing well.”

Speaking to Sputnik on Thursday, the kingdom’s Ambassador to Moscow Rayed Krimly said the two sides were working out the technical details of the contract.

"We signed the contract during the King’s visit. What is now happening is the implementation requires technical details of technology transfers, other technical details between experts of both sides," he said.

"Experts need to finish their discussions, we cannot put a date for the end of discussions, but it is proceeding in a very positive manner," Krimly added.

Last December, Russia said it was working with Saudi Arabia to finalize the agreement to sell the S-400 Triumf, the latest Russian long-range anti-aircraft missile system.

Saudi King Salman bin Abdulaziz Al Saud made a four-day trip to Moscow in March 2017.

During the visit, Russia also agreed to sell Riyadh a Kornet-M anti-armor system, Tos-A1 rocket launcher, AGS-30 grenade launcher, and Kalashnikov AK-103, according to the information office of Russia’s Federal Service for Military-Technical Cooperation.

Riyadh has frequently claimed to have intercepted retaliatory missiles fired from Yemen, but many reports have pointed to the failure of US-made Patriot missile system to fend off the attacks.

In April, The New York Times described the Patriot a “struggling” missile defense system. “The Patriot system has faced recent scrutiny after it failed to protect Saudi Arabia’s capital from missiles fired by Houthi militants in Yemen,” the paper wrote.

A month earlier, Washington-based Foreign Policy magazine published an article, describing the Patriot as “a lemon of a missile defense system,” and casting doubt on the veracity of the kingdom’s claims of neutralizing Yemen's retaliatory missile attacks.

Saudi Arabia relies heavily on the United States in its war on Yemen. Washington has deployed a commando force on Saudi Arabia’s border with Yemen to help destroy arms belonging to Yemen’s popular Houthi Ansarullah movement. The US has also provided logistical support and aerial refueling.

Saudi Arabia and allies invaded Yemen in March 2015 to reinstate Riyadh-allied former officials. The coalition has failed to achieve the goal despite superior military power. Instead, over 14,000 Yemenis have been killed in the war.