Washington - The US government has approved the sale to Saudi Arabia of the advanced Terminal High Altitude Area Defense (THAAD) missile defence system for $15bn, the state department said on Friday."This sale furthers US 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," a statement said.The green light for the deal, which Saudi Arabia has long sought, came one day after King Salman met Russia's President Vladimir Putin and signed a preliminary agreement to look at Moscow's S-400 air defence system.The THAAD - which has already been supplied to Saudi Arabia's neighbours Qatar and the United Arab Emirates - is one of the most capable defensive missile batteries in the US arsenal and comes equipped with an advanced radar system.Its recent deployment by the US military in South Korea to protect against any North Korean strike drew protests from Beijing, who feared its sensors would be able to penetrate into Chinese air space and upset the balance of power.The state department said it would advise Congress that, in Saudi hands, the system would act to stabilise the situation in the Gulf and help defend US forces in the region and their allies, who face a growing Iranian missile capability. "The proposed sale of this equipment and support will not alter the basic military balance in the region," it said.Officials told AFP that the sale, which can go ahead if the US Congress does not object within 30 days, should be seen as part of an overall $110bn arms package that President Donald Trump promised the Saudi kingdom during a visit in May."This potential sale will substantially increase Saudi Arabia's capability to defend itself against the growing ballistic missile threat in the region," a statement said."THAAD's exo-atmospheric, hit-to-kill capability will add an upper-tier to Saudi Arabia's layered missile defence architecture."The main US contractors who will profit from the sale are aerospace giant Lockheed Martin's space systems division and defence contractor Raytheon.