Saudi Arabia is on its way to becoming the second Arab country to use the Terminal High Altitude Air Defence missile system, or Thaad, as the Pentagon confirmed the first $1 billion payment from Riyadh towards the acquisition.
After years of negotiations between Washington and Riyadh, the US Department of Defence confirmed that Lockheed Martin's missile department would fill the order as part of a $110bn arms deal signed by the administration of President Donald Trump.
The payment is the first instalment towards the total Thaad of $15bn. The Pentagon awarded Lockheed a $946 million payment for the sale.
The package includes installation, testing, key personnel and training.
Talks for acquiring Thaad started during the Obama administration in 2011 but concluded under President Mr Trump last year.
The letters formalising the terms of buying 44 Thaad launchers, missiles and related equipment were signed in November, Reuters reported.
The Trump administration has been eager to complete the multibillion-dollar arms sales with Riyadh that the US President announced during his May 2017 visit.
The UAE was the first Arab country to acquire Thaad, a system designed to counter the threat of ballistic missiles, which for Riyadh comes mainly from Iran and the Houthi rebels in Yemen.
The system is already being used in South Korea, Japan and Taiwan to protect against the North Korean threat.
While Oman announced its plan to buy Thaad systems in 2013, the final sale has not been concluded.
Just hours before the payment from Saudi, the US military announced it has sent Thaad parts to Israel for a first joint exercise of its kind. The parts were sent in early March, the US military Command in Europe said.
Israel has long sought to maintain its military advantage in the Middle East, and has intervened in the past to block or downsize deals between Washington and Arab partners.
Meanwhile, Russia is trying to finalise an S-400 defence missile system deal with Turkey, despite the threat of sanctions from US Congress, and to negotiate a new deal with Qatar.