Saudi Arabia signed an agreement last week with the U.S. government for the purchase of 320 more Patriot PAC-3 missiles built by Lockheed Martin Corp , and may accelerate its plans to buy the longer-range THAAD system, a senior Lockheed official told Reuters this week.

The PAC-3 sale is part of a larger possible deal valued at $5.4 billion for a total of 600 PAC-3 missiles that was approved by the U.S. government in July. Such approvals, which do not guarantee a deal will be reached, are followed by negotiations about an actual sale.

Saudi Arabia, a key U.S. ally, is participating in coalition efforts to fight Islamic State militants in Syria, while also battling Iranian-allied Houthi rebels in Yemen, who have already fired Scud missiles against the kingdom. Saudi Arabia also remains concerned about the growing military strength of Iran. […]