SEOUL, March 21 (Reuters) - Korean Air Lines will order 33 Airbus A350 jets in a deal valued at $13.7 billion, its first purchase of the long-haul aircraft family, as it gears up for a pending merger with Asiana Airlines , it said on Thursday.

South Korea's largest carrier will order 27 A350-1000 and six A350-900 planes, which Airbus says use 25% less fuel than similar older generation planes.

Korean Air, which operates a mixed fleet of Boeing and Airbus wide- and narrow-body aircraft, said the order was for its long-term fleet planning as older aircraft retire and to meet sustainability goals.

Korean Air has obtained a green light from competition authorities in 13 of 14 jurisdictions for the purchase of nearly two-thirds of the country's second biggest carrier Asiana for about $1.4 billion. It is awaiting a U.S. decision to finalise the merger, which CEO Walter Cho has said he is confident will be completed by the end of the year.

Cho said in February the carrier will need to simplify its fleet after the Asiana merger as it will have almost every aircraft and engine type available.

"The procurement of the next-generation, eco-friendly A350 is not only aligned with the airline's sustainability efforts, but also is seen as preparation for the integration of Asiana," Korean Air said in a statement. (Reporting by Jack Kim and Lisa Barrington Editing by Ed Davies and Elaine Hardcastle)