At least 21 people were killed when a bus carrying students preparing to sit their annual college entrance exam plunged into a lake in southwest China on Tuesday.

The bus crashed into a guardrail and veered into Hongshan Lake in Guizhou province, leaving 15 others injured, the local emergency management department said in a statement. The incident took place around noon in Anshun city and eyewitnesses said the passengers on the bus included students who were sitting their gruelling "gaokao" examinations that day, state broadcaster CCTV reported.

Security camera footage shared on social media by CCTV showed the vehicle veering across at least five lanes and cutting into oncoming traffic before it ploughed through barriers on the other side of the road. Images from the scene showed search and rescue teams standing by the lake and the bus being fished out of the water in the hours after the crash.

Social media users expressed their condolences on China's microblogging platform Weibo.  "I hope the death toll will not rise anymore. 2020 has truly been full of disasters and difficulties," one user wrote. All those injured in the crash are being treated in hospital. Local authorities pledged an investigation into the cause of the accident.

Guizhou's emergency management department said more than 200 people helped with rescue efforts, including 55 professional divers.