WASHINGTON • The US government has approved the sale to Saudi Arabia of the advanced Terminal High Altitude Area Defence (Thaad) missile defence system for US$15 billion (S$20.5 billion), the State Department said.

"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," it said in a statement on Friday.

The green light for the deal, which Saudi Arabia has long sought, came one day after King Salman bin Abdulaziz al Saud met Russia's President Vladimir Putin and signed a preliminary agreement to look at Moscow's S-400 air defence system.

Thaad - which has already been supplied to Saudi Arabia's neighbours Qatar and the United Arab Emirates (UAE) - is one of the most capable defensive missile batteries in the United States 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 strikes drew protests from Beijing, which 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 help stabilise the situation in the Gulf and defend US forces in the region and their allies, who face a rising Iranian missile capability.

"The proposed sale of this equipment and support will not alter the basic military balance in the region," it said.

The sale can go ahead if Congress does not object within 30 days.

Separately, the US is also scaling back its involvement in some joint military exercises in the Gulf region following a rift between Qatar and its neighbours, a spokesman said.

Asked whether Washington had scaled back military operations with some Gulf Cooperation Council countries due to the rift, US Central Command spokesman Colonel John Thomas said: "We are opting out of some military exercises out of respect for the concept of inclusiveness and shared regional interests."

Saudi Arabia, the UAE and Bahrain, along with Egypt, severed ties with Qatar in June, accusing it of supporting terrorism, in the most serious rift between Washington's Gulf Arab allies.

Doha denies the allegations, saying the sanctions are intended to force it to change its foreign policies.

AGENCE FRANCE-PRESSE, REUTERS