A Boeing 747-400F operated by South Korea's Asiana Airlines was carrying electronic products and liquids when it crashed in sea off Jeju Island.
The electronic products comprised mobile phones, liquid crystal displays and light-emitting diodes, said an Asiana spokesman.
Some of the products had lithium batteries, while the liquids included paints and resins. He added that the cargo was in accordance with IATA regulations.
While there is no immediate evidence that the cargo contributed to the accident, lithium batteries are considered a potentially hazardous cargo owing to the risk they pose of in-flight fire.
The UPS 747-400F which crashed in Dubai in September last year had been transporting lithium batteries when the aircraft suffered a fire in cruise and attempted to divert.
Asiana said its aircraft took off on regular cargo service from Seoul's Incheon airport to Shanghai Pudong airport in China at 02:47 local time. At 04:11 local time the crew reported "control problems" and "was diverted to Jeju International Airport when contact was lost with the aircraft".
While Asiana said the control problems occurred at a height of 7,600ft, the timing of the incident indicates that the aircraft was halfway through the flight and the altitude data may be erroneous, particularly because altitude is measured in metres within Chinese airspace. A figure of 7,600m would equate to 25,000ft, more in line with cruising heights.
At 04:45 local time, a coast guard rescue unit was dispatched. At 05:30 local time, a state of emergency was declared by Asiana. An oil slick, apparently from the aircraft, was discovered at 06:10 local time, and the coast guard rescue unit discovered aircraft debris at 06:50 local time.
"We are still waiting for a report from the coast guard and our staff," said the spokesman. "Some debris and oil has been spotted floating in the sea, but the aircraft has not been found yet."
The spokesman said that there were two crew, a pilot and co-pilot, in the missing aircraft.
The airframe was built in 2006 and has clocked 26,300h, as shown in Flightglobal's ACAS data. The aircraft is powered by four GE CF6-80C2 engines.
Asiana operates five 747-400Fs, four 747-400BDSFs, and two 747-400 combis.