Bird feathers and bloodstains were found in both engines of the Jeju Air plane that crashed in December, according to a preliminary investigation released Monday.

The Boeing 737-800 was flying from Thailand to Muan in South Korea on December 29 when it crash landed and exploded into a fireball after slamming into a concrete barrier.

It was the worst aviation disaster on South Korean soil, killing 179 of the 181 passengers and crew.

South Korean and American investigators are still probing the cause of the disaster, with a bird strike, faulty landing gear and the runway barrier among the possible issues.

Both engines recovered from the crash site were inspected, and bird bloodstains and feathers were "found on each", the report said.

"The pilots identified a group of birds while approaching runway 01, and a security camera filmed HL8088 coming close to a group of birds during a go-around," the report added, referring to the Jeju jet's registration number.

It did not specify whether the engines had stopped working in the moments leading up to the crash.

DNA analysis identified the feathers and blood as coming from Baikal teals, migratory ducks which fly to Korea in winter from their breeding grounds in Siberia.

After the air traffic control tower cleared the jet to land, it advised the pilots to exercise caution against potential bird strikes at 8:58 am, the report said. Just a minute later, both the voice and data recording systems stopped functioning.

Seconds after the recording systems failed, the pilots declared mayday due to a bird strike and attempted a belly landing.

The Jeju plane exploded in flames when it collided with a concrete embankment during its landing, prompting questions about why that type barricade was in place at the end of the runway.

Last week, authorities said they would replace such concrete barriers at airports nationwide with "breakable structures".

The captain had over 6,800 flight hours, while the first officer had 1,650 hours, according to the report. Both were killed in the crash, which was survived only by two flight attendants.