Qatar and the United Arab Emirates have requested the sale of up to $7.6bn in Lockheed Martin Corp missile-defence systems to counter perceived threats and lower their dependence on US forces, the Pentagon announced.
The Defence Security Cooperation Agency (DSCA), which oversees foreign arms sales, formally notified lawmakers on Friday that it had approved the possible sales, which come against the backdrop of heightened tensions with Iran.
The notifications were posted to the agency's website late on Monday.
Lawmakers now have 30 days to block the potential sales although such action is rare since deals are carefully vetted with lawmakers weeks before the notifications are posted.
The sale is part of Washington's ongoing effort to deepen its co-operation with Gulf nations on missile defence and increase pressure on Iran over its nuclear program.
Lockheed told reporters in August that Saudi Arabia and its closest regional partners in the Gulf Co-operation Council had shown interest in the company's Terminal High-Altitude Area Defence (THAAD) weapon systems.
Secretary of State Hillary Clinton met GCC officials in September and US officials said initial missile-defence sales could be announced soon.
The GCC is a political and economic alliance linking Saudi Arabia, Kuwait, the United Arab Emirates, Qatar, Bahrain and Oman.
Washington has been working with Gulf states on a bilateral basis, not as a group, to boost the range of radar coverage and related capabilities across the Gulf for the earliest possible defence against any missiles fired by Iran.
The United States and its allies say Iran is seeking nuclear weapons capability under the cover of a civil program. Iran denies this, but has been hit with a series of international sanctions over its nuclear work.
On Monday, the Pentagon said Qatar had requested the possible sale of two THAAD fire units, 12 launchers, 150 interceptors, parts, training and logistical support for an estimated cost of $6.5bn.
The UAE, which signed an initial order for $1.96bn of THAAD weapons systems in December, requested an additional 48 THAAD missiles, nine launchers and other equipment valued at $1.135bn, according to the DSCA notification.
It said the proposed sale would contribute to the foreign policy and national security of the United States by helping two countries that have been and remain key forces "for political stability and economic progress in the Middle East."
Raytheon Co is another key contractor on the program.
THAAD is a US Army system designed to shoot down short-, medium- and intermediate-range ballistic missiles with an interceptor that slams into its target.
It can accept cues from Lockheed's Aegis weapons system, satellites and other external sensors, and works in tandem with the PATRIOT Advanced Capability-3 terminal air-defence missile. THAAD includes its own radar along with interceptors and communications and fire control units.
US officials have said their ultimate goal is a regional shield that can be coordinated with US systems, a system similar to Washington's drive to expand missile defence to protect NATO's European territory against ballistic missiles that could be fired by Iran.
THAAD is part of a layered missile shield being built to defend the United States and its friends and allies against ballistic missiles of all ranges and in all phases of flight. The system is being optimised against Iran and North Korea.