While continuing to look for survivors after a China Eastern Airlines flight crashed in South China's Guangxi Zhuang autonomous region on Monday, over 2,000 search and rescue workers on Tuesday have intensified efforts to recover the black box of the aircraft to investigate the cause of the tragedy.

Flight MU5735 left Kunming, capital of Yunnan province, at 1:11 pm and was scheduled to arrive in Guangzhou, Guangdong province at 3:05 pm. Air traffic controllers lost track of the plane over Wuzhou, Guangxi. The flight was carrying 123 passengers and nine crew members.

Along with search and rescue workers, drones have been deployed at the crash site to search for more evidence over an area of nearly 680,000 square meters. Zhu Xiaodong, a rescuer with a drone rescue center in Guangzhou, said at the site it's difficult to carry out search work as there are only a few paths leading to the site. The drones' thermal imaging function will help locate the black box, which records flight data and cockpit conversation.

According to video footage released by CCTV around noon on Tuesday, as the search and rescue team continued to collect debris, they also found wallets, train tickets, identification cards and bank cards at the crash site in a forest near Tengxian county's Molang village.

A hand-written note about peace buckles — a traditional type of Chinese jewelry — discovered at the site has made many more upset about the tragedy as the buckles represent people's wishes for a peaceful and long life.

Experts from the public security department and the Civil Aviation Administration of China have entered the crash site to search for physical evidence, confirm the identity of the missing persons and find out the possible cause of the accident. No information about the passengers and crew members was available by Tuesday evening.

Much of the debris from the aircraft is in small pieces, said Li Chenbin, who heard a loud bang echoing across the mountains at around 2:20 pm on Monday.

"I heard a loud bang while working on the mobile communication network nearby," said Li, a technician with China Mobile's Guangxi branch. "I then rushed to the site with my colleagues and saw the aircraft debris, much of which was in small pieces. It was scattered all over the mountains. The biggest piece I saw was part of a wing with China Eastern's logo."

Without a stable electricity supply, search and rescue teams have been using emergency lights and head lights to work overnight, Li said. He stayed near the site to maintain the mobile communication network that is critical for search and rescue efforts.

As the crash site is surrounded by mountains on three sides with only a narrow road that leads to it, it's been difficult to carry out search and rescue work. Local villagers have been using motorbikes to carry supplies to the crash site.

On Monday night, Vice-Premier Liu He and State Councilor Wang Yong traveled to Wuzhou to guide the rescue efforts, the aftermath and the investigation into the cause of the crash.

UN Secretary-General Antonio Guterres expressed his condolences to the victims and the Chinese government, said Stephane Dujarric, spokesperson for the UN chief. The International Civil Aviation Organization also said it is saddened by the tragic loss and expresses its deepest condolences to all affected.

Xinhua contributed to this story.