Crematoriums across China are straining to deal with an influx of bodies as the country battles a wave of Covid cases that authorities have said is impossible to track.

Cases are soaring across China, with hospitals struggling and pharmacy shelves stripped bare in the wake of the government's sudden decision to lift years of lockdowns, quarantines and mass testing.

The United States has warned the outbreak is now of concern to the rest of the world, given the potential for further mutations and the size of China's economy.

From the country's northeast to its southwest, crematorium workers told AFP they are struggling to keep up with a surge in deaths.

In Chongqing — a city of 30 million where authorities this week urged people with mild Covid symptoms to go to work — one worker told AFP their crematorium had run out of space to keep bodies.

"The number of bodies picked up in recent days is many times more than previously," a staffer who did not give their name said.

"We are very busy, there is no more cold storage space for bodies," they added.

"We are not sure (if it's related to Covid), you need to ask the leaders in charge."

In the southern megapolis of Guangzhou, an employee at one crematorium in Zengcheng district told AFP they were cremating more than 30 bodies a day.

"We have bodies assigned to us from other districts. There's no other option," the employee said.

Another crematorium in the city said they were also "extremely busy".

"It's three or four times busier than in previous years, we are cremating over 40 bodies per day when before it was only a dozen or so," a staffer said.

"The whole of Guangzhou is like this," they added, stressing that it was "hard to say" whether the surge in bodies was linked to Covid.