The Pentagon's Defense Security Cooperation Agency says Seoul has requested a possible $1.2 billion sale of four Northrop Grumman Corp RQ-4 "Global Hawk" remotely piloted aircraft with enhanced surveillance capabilities.
The security agency told US lawmakers South Korea needed such systems to assume top responsibility for intelligence-gathering from the US-led Combined Forces Command as scheduled in 2015.
"The proposed sale of the RQ-4 will maintain adequate intelligence, surveillance, and reconnaissance capabilities and will ensure the alliance is able to monitor and deter regional threats in 2015 and beyond," it said in a notice.
The United States has agreed with Seoul to turn over the wartime command of Korean troops later this decade. Current arrangements grew from the US role in the 1950-1953 Korean War that repelled a North Korean takeover of the South.
Seoul has shown interest in the high-altitude, long-endurance Global Hawk platform for at least four years.
The system, akin to Lockheed Martin Corp's U-2 spy plane, may be optimised to scan large areas for stationary and moving targets by day or night and despite cloud cover.
It transmits imagery and other data from 18,300 meters at near real-time speed, using electro-optical, infrared and radar-imaging sensors built by Raytheon Co.
The possible sale has been held up by discussions involving price, aircraft configuration and a go-slow on release of such technology subject to a voluntary 34-nation arms control pact.
The Defence Department began informally consulting Congress on the possible Global Hawk sale in the summer of 2011, only to withdraw it pending further work on the make-up of the proposed export to Seoul amid lawmakers' arms-control concerns.
The formal notification to Congress came less than two weeks after a North Korean space launch of a satellite atop a multi-stage rocket, a first for the reclusive state, widely seen as advancing its ballistic missile program.
A White House statement denounced the December 12 launch as a "highly provocative act" that would bear consequences for violations of United Nations resolutions.
The North is banned from testing missile or nuclear technology under international sanctions imposed after its 2006 and 2009 nuclear weapons tests.
In October 2008, then-US Defence Secretary Robert Gates told reporters the United States was "very sympathetic" to South Korea's interest in Global Hawk. But he cited issues that had to be overcome because of the so-called Missile Technology Control Regime, or MTCR.
South Korea's possible Global Hawk purchase would mark the system's first sale in the Asia-Pacific region. It has already been sold to the North Atlantic Treaty Organization (NATO).
Northrop Grumman officials have said Australia, Japan and Singapore each have shown interest in buying Global Hawk systems.