The US government has approved the possible sale of a THAAD anti-missile defence system to Saudi Arabia at an estimated cost of $15 billion, the Pentagon said on Friday, citing Iran among regional threats.

The approval opens the way for Saudi Arabia to purchase 44 Terminal High Altitude Area Defence (THAAD) launchers and 360 missiles, as well as fire control stations and radars.

“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,” the Pentagon’s Defence Security Cooperation agency said in a statement.

Saudi Arabia and the US are highly critical of what they consider to be Iran’s aggressive behaviour in the Middle East.

Iran also has one of the biggest ballistic missile programmes in the Middle East, viewing it as an essential precautionary defence against the US and other adversaries, primarily Gulf Arab states and Israel.

THAAD missile systems are deployed to defend against ballistic missile attacks.

Saudi-owned al Arabiya television reported that the kingdom had agreed to buy Russian S-400 surface-to-air missile systems, an announcement that came as Saudi King Salman made during his visit to Russia, the first by a Saudi monarch.

US military sales to Saudi Arabia have come under increased scrutiny over the Saudi-led coalition’s war in Yemen.

The situation in Yemen Show all 14 1 /14 The situation in Yemen The situation in Yemen Houthi supporters trample on a US flag during a gathering mobilizing more fighters into several Yemeni battlefronts, in Sana'a, Yemen EPA The situation in Yemen People carry the coffins of men, who were killed in the recent Saudi-led airstrikes during their funeral, in the Old City of Sanaa, Yemen AP The situation in Yemen Pro-government fighters give food to Yemeni children on the road leading to the southwestern port city of Mokha. Yemeni rebels are putting up fierce resistance in a key Red Sea port city where they are encircled by pro-government force Getty Images The situation in Yemen A Yemeni stands in front of a graffiti protesting US military operations in war-affected Yemen, in Sana'a, Yemen. According to reports, US Special Forces troops allegedly disembarked from US helicopters in the Yemeni town of Yakla and attacked several houses belonging to members of the terrorist group Al-Qaeda, killing three high-ranking Al-Qaeda members and nine civilians, six women and three children. One American serviceman has been killed and three injured in the attack EPA The situation in Yemen US Special Forces troops allegedly disembarked from US helicopters in the Yemeni town of Yakla and attacked several houses belonging to members of the terrorist group Al-Qaeda, killing three high-ranking Al-Qaeda members and nine civilians, six women and three children. One American serviceman has been killed and three injured in the attack EPA The situation in Yemen A Yemeni female fighter supporting the Shiite Huthi rebels, and carrying weapons used for ceremonial purposes, takes part in an anti-Saudi rally in the capital Sanaa Getty Images The situation in Yemen Yemeni female fighters supporting the Shiite Huthi rebels, and carrying weapons used for ceremonial purposes, take part in an anti-Saudi rally in the capital Sanaa Getty Images The situation in Yemen A boy shouts slogans next to pro-Houthi fighters, who have been injured during recent fighting, during a rally held to honour those injured or maimed while fighting in Houthi ranks in Sanaa, Yemen Reuters The situation in Yemen Balls of fire and smoke rise from a Houthi-held military camp following alleged Saudi-led airstrikes, in Sana'a, Yemen EPA The situation in Yemen Yemenis search under the rubble of damaged houses following reported Saudi-led coalition air strikes on the outskirts of the Yemeni capital Sanaa Getty Images The situation in Yemen A Yemeni boy looks on as Yemenis search under the rubble of damaged houses following reported Saudi-led coalition air strikes on the outskirts of the Yemeni capital Sanaa Getty The situation in Yemen A Yemeni boy sits amidst the rubble of damaged houses following reported Saudi-led coalition air strikes on the outskirts of the Yemeni capital Sanaa AFP/Getty The situation in Yemen Marine One with US President Donald Trump flies with a decoy and support helicopters to Dover Air Force Base in Dover, Delaware, for the dignified transfer of Navy Seal Chief Petty Officer William 'Ryan' Owens who was killed in Yemen Getty Images The situation in Yemen US President Donald Trump aboard the Marine One to greet the remains of a US military commando killed during a raid on the al Qaeda militant group in southern Yemen on Sunday, at Dover Air Force Base, Delaware, US Reuters

Riyadh and its allies have been bombing the Iran-aligned Houthis in Yemen since the Houthis seized much of the country’s north in 2015.

Riyadh insists the coalition is fighting terrorists and supporting Yemen’s legitimate government but the office of the UN human rights chief has said Saudi-led air strikes cause the majority of civilian casualties including many children.

Lockheed Martin is the prime contractor for the THAAD system, with Raytheon playing an important role in the system’s deployment.

The United States deployed THAAD to South Korea this year to guard against North Korea’s shorter-range missiles, drawing fierce criticism from China, which says the system’s powerful radar could probe deep into its territory.