The search continued for the flight data recorder.
Debris from the jetliner including engine blades, horizontal tail stabilisers and other wing remnants was concentrated within 30 metres of the main impact point, which was 20 metres deep.
One 1.3 metre-long fragment suspected to be from the plane was found about 10 km away, prompting a significant expansion of the search area, officials told a news briefing.
No survivors have been found, and experts have said it was all but impossible that anyone could survive such an impact.
Flight MU5735 was en route from the southwestern city of Kunming to Guangzhou on the coast when the plane suddenly plunged from cruising altitude at about the time when it should have started its descent to its destination.
The investigation is being led by China but the United States was invited to take part because the plane was designed and manufactured there.
However, the U.S. National Transportation Safety Board said on Wednesday it had not determined if investigators would travel to China given strict visa and quarantine requirements, and Chinese officials declined to say whether or when NTSB officials would be invited.
"Our work priority is still on search and rescue, and at the same time, carrying out evidence collection and fixation work in the early stage of accident investigation," said Zhu Tao, the CAAC's head of aviation safety.
"However, when we enter the accident investigation stage, we will invite relevant parties to participate in the accident investigation according to relevant regulations," he said.
According to flight tracking website FlightRadar24, the plane briefly appeared to pull out of its nosedive, before plunging again into a heavily forested slope in the mountainous Guangxi region.
Authorities said the pilots did not respond to repeated calls from air traffic controllers during the rapid descent.
It was too early to determine the cause of the crash, which experts say are usually the result of a combination of factors.
"The difficulty now is that we are eager to search for survivors as soon as possible, but our work requires us to search carefully and slowly," Huang Shangwu, deputy director of the Combat Training Office of the Guangxi Fire Rescue Corps said at the site.
Search teams used thermal imaging cameras and life detection devices as well as drones.
"The search area is really large, plus the two days of rain make the path very slippery," said Zhou, among the more than 1,600 people involved in search operations on Thursday.
The flight's captain had 6,709 hours flying experience, while the first and second officers had 31,769 hours and 556 hours, respectively, a China Eastern official said on Wednesday. One co-pilot was an observer to build up experience, the airline said, without disclosing the names of the pilots.
Phoenix Weekly magazine cited an aviation expert who identified the captain as Yang Hongda, the son of a former China Eastern captain, and the first officer as Zhang Zhengping, a pilot with 40 years of experience who mentored other pilots.
The Southern Weekly newspaper reported Yang, 32, had a one-year-old daughter, while Zhang, 59, was a veteran pilot with an impeccable safety record and had been expected to retire this year. Another media outlet, Jimu News identified the less experienced second officer as Ni Gongtao, 27.
China Eastern did not respond immediately to a request for comment on the reports.