All 132 people aboard the plane that crashed into a mountainside in southern China this week have been confirmed dead, the country's civil aviation authority said Saturday.

Dozens of victims' relatives have been waiting for days as rescue teams combed heavily forested slopes for plane debris and signs of survivors from Monday's crash near the city of Wuzhou, Guangxi province. 

"All 123 passengers and nine crew members of flight MU5735 of China Eastern airlines have been killed on board on March 21," Hu Zhenjiang, deputy director-general of the Civil Aviation Administration of China, told a press conference.

"The identity of 120 victims has been determined by DNA identification."

Aviation officials confirmed they had found a black box they believed to be the cockpit voice recorder, which should provide important clues to the cause of the crash.

The cause has mystified aviation authorities, who have scoured rugged terrain for clues in what is almost certain to be China's deadliest plane crash in nearly 30 years.

The disaster provoked an unusually swift public response from President Xi Jinping, who ordered a probe into its cause as aviation authorities vowed an extensive two-week check-up of China's vast passenger fleet.

The safety message has rippled out across sectors since the crash. 

A notice from the State Council and Ministry of Emergency Management on Wednesday called for industries across the board to "rectify potential safety hazards".