The sale of the anti-aircraft systems to U.S. allies - Turkey and Saudi Arabia - was criticized in Washington, but did not affect the outcome of the deals.
The king has been visiting Russian President Vladimir Putin in talks over oil and Syria, Saudi's al Arabiya television reported.
When asked whether a possible US deal with Riyadh might affect the Russian arrangement, Kremlin spokesman Dmitry Peskov, on a conference call with reporters, said talks had been progressing well.
The department said the sale of the equipment to the Saudi people would help provide a balance to a relatively unstable environment in the Gulf and to help the U.S. forces enlarge its allied grip on the region.
Russia has agreed to supply Saudi Arabia with a range of sophisticated modern weapons including S-400 anti-aircraft missile systems, the Russian Federal Service for Military-Technical Cooperation said Monday.
Barron bill would change Columbus Day to 'Indigenous People's Day'
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Saudi Arabia is a longstanding USA ally in the Middle East and its arms deals with Moscow have caused disquiet in Washington.
Earlier, the Bloomberg news agency published information with reference to a press release from the Saudi Arabian Military Industries (SAMI), according to which Moscow and Riyadh signed contracts providing for the purchase of S-400 air defense missile systems.
The deputy commander of Russia's Aerospace Force, Viktor Gumenny, said that the S-400 systems began to be armed with missiles capable of hitting targets in near space.
Saudi Foreign Minister, Adel Al-Jubeir, has said that United States policy with Russian Federation will not affect Riyadh's plans for cooperation with Moscow.
"Therefore we are confident that this cooperation should not be a worry for anyone".