The approval has now opened the way for Saudi Arabia's purchasing of the 44 Terminal High Altitude Area Defense (THAAD) launchers and 360 missiles, as well as fire control stations and radar, says a report in Reuters.
Russian Federation and Saudi Arabia have agreed on the supply of S-400 anti-aircraft missile systems, as well as a number of other weapons, including the Kalashnikov AK-103.
"This sale furthers USA national security and foreign policy interests, and supports the long-term security of Saudi Arabia and the (Persian) Gulf region in the face of Iranian and other regional threats", the Pentagon's Defense Security Cooperation agency claimed in a statement.
The S-400 is Russia's most advanced long-range anti-aircraft missile system and can carry three types of missiles capable of destroying targets, including ballistic and cruise missiles.
A Saudi official has commented on Moscow's role in the Middle East following the "landmark" visit of the Saudi King to Russian Federation last week.
"This sale furthers USA national security and foreign policy interests, and supports the long-term security of Saudi Arabia and the Gulf region in the face of Iranian and other regional threats", the statement said.
Both Washington and Saudi Arabia accuse Tehran of aggressive and disruptive behavior in the region.
"THAAD's exo-atmospheric, hit-to-kill capability will add an upper-tier to Saudi Arabia's layered missile defense architecture". "This visit is the beginning of strategic cooperation", al Harithi said.
U.S. THAAD system equipment in Seongju, South Korea, last month.
Maria Vorobyova, an official at a Russian government agency dealing with military and technical cooperation, was cited earlier on Monday as saying that a firm agreement had been reached with Saudi Arabia on the S-400s.