At least 21 people were killed and 15 injured after a bus fell into a lake in the city of Anshun, Guizhou province, on Tuesday, local authorities said.

Among the passengers were four local students, People's Daily reported. One of the four students died and the other three are still receiving medical treatment.

The bus, which was reported to be "full of people", broke through a guardrail on the road and fell into the Hongshan Reservoir in the city's Xixiu district at around 12:30 pm.

Local police are conducting an investigation, online news portal ThePaper.cn reported. The cause of the accident has not yet been determined.

Shen Yiqin, Guizhou's provincial governor, visited the victims on Tuesday afternoon and emphasized that saving lives should be the priority.

The Guizhou Emergency Management Office said on its Weibo account on Tuesday afternoon that the number of casualties were unknown and rescue work is ongoing.

The Ministry of Emergency Management responded quickly to the accident, sending a special work team to Anshun on Tuesday afternoon to support rescue efforts.

Huang Ming, vice-minister of emergency management, urged local departments to focus efforts on rescuing the people. He also emphasized that the number of casualties and the cause of the accident should be confirmed as soon as possible.

"The department in charge of the bus and the person who caused accident should be investigated for legal responsibility according to law," Huang said. "Potential risks and transportation system loopholes should be checked as preventive measures."

The Ministry of Public Security also sent a team to Anshun to help investigate the incident.

According to the Guizhou Emergency Management Office, the bus was salvaged from the reservoir at about 3 pm. At least 19 fire trucks, 21 rubber boats and 97 rescuers including 17 divers have been dispatched to the site to look for survivors.

The blue electric bus, which was running route No 2 in the city, was purchased last year, according to the China Economic Net.