State media Xinhua said soil at the crash site had absorbed a lot of water, adding “uncertainty” to the rescue.
No survivors have so far been found from China Eastern flight MU5735 which had 132 passengers when it crashed in the hills in southern China.
Investigators don’t know why the plane plummeted out of the sky on Monday.
There has been an outpouring of grief in China, where families of passengers and crew are still waiting for news.
Hundreds of responders have been scouring the steeply forested slopes in Wuzhou where debris from the plane was strewn after it broke apart and set fire to the hillside.
There had been no official word on casualties until China’s Civil Aviation Administration held its news briefing on Tuesday evening, some 36 hours after the disaster.
“Up to now, search and rescue work has not found any survivors,” Zhu Tao, aviation safety office director at the CAA, told reporters. “Given the information currently available, we still do not have a clear assessment of the cause for the crash.”
Air controllers had repeatedly called the aircraft during its descent but had received no response, he added.
Rescuers have so far found parts of the 737’s burnt wreckage. State broadcasters showed images of the charred remains of letters, bags, wallets and identity cards belonging to those on board.
Meanwhile, the families and friends of the 123 passengers and nine crew have gathered at each end of the flight – with relatives visiting China Eastern’s offices in Yunnan province and waiting at Guangzhou International Airport.
The China Eastern Airlines flight from Kunming, the capital of Yunnan, had been due to land in Guangzhou on Monday afternoon.
China Eastern has grounded all its Boeing 737-800s and set up a hotline for people seeking information on those on board.
Authorities have yet to identify passengers and crew members, but some relatives have spoken to local media or shared their grief online.
One woman reported the loss of her newly-wed husband on her WeChat account. Her earlier posts included videos of the couple’s holiday trips.
Other passengers included a group of six people, one of them a teenager, who were on their way to Guangzhou to attend a funeral, a local newspaper reported.