"THAAD's exo-atmospheric, hit-to-kill capability will add an upper-tier to Saudi Arabia's layered missile defense architecture".
Despite increasing criticism over the United States' military sales to Saudi Arabia, the US State Department has paved the way for the potential purchase of controversial - and expensive - military equipment.
The king has been visiting Russian President Vladimir Putin in talks over oil and Syria, Saudi's al Arabiya television reported.
The Kremlin has confirmed the sale of the Russian-made S-400 air defense missile system to Saudi Arabia.
It is expected the sale will beef-up security in the nuclear-hungry Middle East.
Last month Turkish President Recep Tayyip Erdogan said that Turkey had signed a deal with Russian Federation to buy S-400 missile defense systems.
The S-400 is a very capable missile-defense system, somewhat comparable to the US's MIM-104 Patriot, according to CSIS.
He said that Riyadh was set to cooperate with Moscow in the fight against terrorism.
However, despite the difficulties, Saudi Arabia has chose to diversify its military suppliers, as it understands it can't rely on Washington so heavily.
"They pose a threat for our countries and for other states, from where they came from".
Washington deployed THAAD, which is used to guard against ballistic-missile attacks, to South Korea this year for defense against North Korea. Saudi Arabia's neighbours, Qatar and the United Arab Emirates, have also been supplied THAAD systems.
Meanwhile, Lockheed Martin Saudi Arabia's chief executive, Alan Chinoda, said that the arms deal brokered during the US President Donald Trump's visit last May to the oil-kingdom encompasses the settings of a Saudi-US partnership and encourages the exchange of intelligence.