Saudi Arabia will reportedly buy Lockheed Martin’s $15 billion missile defence system in a deal that included a personal call between President Donald Trump and Saudi King Salman.

Reuters cited the State Department as saying that Saudis and US officials signed letters of offer and acceptance documents, formalizing terms for Saudi Arabia’s purchase of 44 Terminal High Altitude Area Defense (THAAD) launchers, missiles and related equipment.

It said the THAAD deal had been under discussion since December 2016 and is now completed, quoting a US Department of State spokesman.

The deal, first reported by CNBC, supports the “long-term security of Saudi Arabia and the Gulf region in the face of the growing ballistic missile threat from the Iranian regime and Iran-backed extremist groups".

US arms exports surged 25 percent over the five years to 2017, according to data released earlier this year by the Stockholm International Peace Research Institute.

With the Middle East accounting for 49 percent of total US arms exports, Saudi Arabia was by far its biggest customer.

Total arms imports by the kingdom rose 225 percent over the past five years, making it the world’s second-largest importer overall.