Three people remained missing as of Monday evening after the ground gave way in downtown Guangzhou, Guangdong province, according to local authorities. Search and rescue operations are underway.

The collapse happened on Sunday morning at a construction site along metro line 11 in the city's Tianhe district, causing a street-cleaning vehicle and a scooter to plunge into a watery pit some 38 meters deep.

Two people in the vehicle were identified as a 51-year-old father and his son, 26, from Shaoyang, Hunan province.

The identity of the scooter driver remained unknown, according to Sun Chengwei, general manager of the Guangzhou Metro Group construction department.

"So far, police have not received any reports from relatives of the man driving the electric scooter," Sun said.

Rescue efforts were hampered by additional collapses of mud and sand near the scene.

Construction workers in a nearby subway tunnel have been evacuated. Surrounding water, gas, electric and telecom facilities as well as nearby roads and buildings were being closely monitored.

"We have to ensure the safety of rescue workers and nearby buildings so we reinforced the round sides of the pit with concrete," Sun said.

A large steel casing was already installed in the pit to better ensure the safety of rescue workers, Sun added.

After the collapse, a viaduct above the collapse was also being closely monitored before reopening for use on Monday morning.

The accident did not affect functioning metro lines but did cause major traffic congestion on roads near the collapse site on Sunday.

"When we arrived at the site, we did not see any people and vehicles," said Lin Yi, technical director of the Guangzhou Blue Sky Rescue Association.

Lin said electricity, water and gas facilities were already shut down when the rescue team arrived at the scene.

"Multiple collapses happened, with sand and mud quickly flowing down into the pit," said firefighter Chen Junqing.

More than 1,100 rescue workers have participated in rescue efforts. The cause of the accident is still under investigation.