"We will not let anyone spread the ideology of hatred, to finance that kind of ideology or terrorism", he said.
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.
Saudi Arabia is a longstanding USA ally in the Middle East and its arms deals with Moscow have caused disquiet in Washington.
Contacts between Moscow and Riyadh over the supplies of Russian S-400 air defense systems to Saudi Arabia have achieved a very good preliminary result, Russian presidential spokesman Dmitry Peskov said Monday. "The contacts over implementing this deal have been very positive", TASS quoted Peskov as saying.
The king has been visiting Russian President Vladimir Putin in talks over oil and Syria, Saudi's al Arabiya television reported. US Congress has 30 days to object.
The U.S. government has approved the sale to Saudi Arabia of the advanced Terminal High Altitude Area Defense (THAAD) missile defense system for $15 billion, the State Department said on October 6.More news: Very humid Monday with leftover rain from Nate, mainly this morning
The S-400 is a very capable missile-defense system, somewhat comparable to the US's MIM-104 Patriot, according to CSIS.
Iran maintains one of the largest ballistic-missile programs in the Middle East, seeing it as a key element of defense against the United States and other opponents, including Israel and Gulf Arab states.
This sale is likely to further U.S. national security and foreign policy interests and support the long-term security of Saudi Arabia and the Gulf region.
However, despite the difficulties, Saudi Arabia has chose to diversify its military suppliers, as it understands it can't rely on Washington so heavily.
The US has already deployed THAAD to South Korea in defence of North Korea's short-range missiles, which has angered China for interfering.