A South Korean oil refinery has shipped a large amount of heavy fuel oil to North Korea as part of an aid package pledged by the U.S. under a six-party nuclear agreement.

The agreement requires the North to disable its nuclear facilities in return for energy aid to be provided by the five other countries.

The GS Caltex refinery said the 21-thousand-ton shipment left the southern port of Yeosu on Sunday and will be unloaded in the North Korean ports of Songrim and Sonbong.

GS Caltex won the contract to provide the oil in an international bid carried out by the U.S. Agency for International Development, a government agency that provides economic and humanitarian assistance around the world. The bid picked two shipping companies which will deliver 50-thousand tons of heavy fuel oil promised by the U.S.

Dutch oil trading firm Vitol will deliver the remaining 29-thousand tons.