South Korea is not necessarily committed to buying US Global Hawk surveillance drones, a spokesman said Wednesday, after the Pentagon requested congressional permission for such a sale.

Seoul's Defense Acquisition Program Administration (DAPA) said it would decide early next year whether to buy the high-altitude unmanned aerial vehicles made by Northrop Grumman that have come with a higher than expected price tag, at $1.2 billion for four of the drones.

"We will decide whether to proceed with the purchase plan only after we receive a letter of intent and carefully study the sale's terms," a DAPA spokesman told AFP.

Yonhap news agency quoted an unidentified top government official as saying Seoul could consider other choices, such as Boeing's Phantom Eye and the California-based AeroVironment Global Observer.

The US Defence Security Cooperation Agency (DSCA) said Tuesday it had notified Congress of a possible sale of four remotely-piloted Global Hawk aircraft.

"We've never said we would buy no other surveillance drones than Global Hawks," the South Korean official was quoted as saying by Yonhap after the price tag suggested by DSCA appeared to be prohibitively high.

"Competing drones could be considered", the official said.

"Negotiations would have to start anywhere below 800 billion won (745 million dollars) in total, as was suggested by the US side last October," the official added.

South Korea relies heavily on its ally the United States for intelligence gathering and surveillance capabilities over nuclear-armed North Korea.