The Pentagon has formally informed Congress of a plan to sell four Global Hawk surveillance drones to South Korea, a related agency said Monday.

The deal under the Foreign Military Sales (FMS) program, if sealed, would be worth up to $1.2 billion (1.28 trillion won), according to the Defense Security Cooperation Agency (DSCA) affiliated with the Department of Defense.

It includes four RQ-4 Block 30 Global Hawk remotely-piloted aircrafts equipped with the Enhanced Integrated Sensor Suites (EISS), associated equipment, parts, training and logistical support, added the DSCA.

Such a notification to Congress is mandatory for FMS sales.

Congress is expected to approve the plan, a diplomatic source said.

"The Republic of Korea (South Korea) needs this intelligence and surveillance capability to assume primary responsibility for intelligence gathering from the U.S.-led Combined Forces Command in 2015," the agency said in a press release.

It was referring to Seoul's move to take over wartime operational control of its troops from Washington.

The South Korean military has long sought to introduce the high-altitude unmanned aerial vehicles made by Northrop Grumman based in Virginia.

The Global Hawk drone carries a cloud-penetrating radar, a high-resolution electro-optical digital camera and an infrared sensor, enabling it to detect 30-centimeter-long objects while flying 20 kilometers high.

The U.S. was apparently reluctant to sell it to South Korea, but Washington has changed its position amid defense budget cuts.

The U.S. military has scaled back the purchases of Global Hawks, ratcheting up the need for exports.

Critics question the capability of the Block 30 Global Hawk for its price. They say the Block 30 version isn't as capable as Lockheed Martin's older, manned U-2 spy plane.

The U.S. Air Force hopes to buy the more advanced Block 40 vehicles to replace the Block 30 ones. (Yonhap News)