China has completed the main search and rescue work at the site of a China Eastern Airlines plane crash last week, and plans to complete a preliminary report within 30 days of the event, an aviation official said on Thursday.

A final report into the crash of the Boeing 737-800 will be completed and made public after the investigation is concluded, Civil Aviation Administration of China (CAAC) safety head Zhu Tao told reporters.

The plane crashed into a mountainside in southern China on March 21, killing all 132 people onboard.

Under international rules, a 30-day report must be lodged with the U.N. aviation agency ICAO but it does not need to be public. A final report is due within a year of the crash, though sometimes it can take longer.

Zhu said authorities have done preliminary analyses of the crash, such as predicting the possible trajectory, position and impact force of the airplane when it hit the ground and reading data from air traffic control radar systems.

CAAC investigators are also working to decode the data from both the black boxes, he added.

Investigators from the U.S. National Transportation Safety Board are set to travel to China to support the CAAC's investigation into mainland China's deadliest aviation disaster in 28 years.

Over 40,000 pieces of aircraft wreckage and debris have been found and a majority of the pieces have been transported to a hangar, Zhu said.

(Reporting by Stella Qiu in Beijing and Jamie Freed in Sydney Editing by Shri Navaratnam and Raju Gopalakrishnan)