China Railway Group expressed on Sunday condolences and pledged a thorough investigation after an under-construction bridge accident in Qinghai province killed 12 people and left four missing.

The incident occurred at around 3:10 am on Friday, when a steel cable snapped during a tensioning operation at the site of the bridge on the Qinghai section of the Xining-Chengdu Railway in Jainca county, Huangnan Tibetan autonomous prefecture.

The failure caused a 108-meter steel girder arch rib to collapse. There were 16 workers at the site at the time of the accident.

The group said its wholly owned subsidiary, China Railway Major Bridge Engineering Group Co Ltd, which is responsible for the project, immediately activated its emergency response plan. And the company has assisted local authorities in emergency rescue, search and recovery efforts, and subsequent investigations.

"We express deep condolences for the victims and our sincere apologies to their families," the company said in the statement, adding that it would "draw profound lessons, implement corporate responsibility, and properly handle related follow-up work".

The bridge measures 1,596.2 meters in length with a contract value of about 436 million yuan ($61 million). The company noted that the accident would not have a major impact on its overall operations and performance.

On Saturday, an investigation team has been set up by the provincial government, comprising four specialized groups focused on coordination, technical analysis, management review and rescue evaluation.

A report on the causes and responsibilities of the accident, along with recommendations for disciplinary and legal actions, will be released promptly.

The Office of the Work Safety Commission under the State Council has also dispatched a working group together with related authorities to guide the investigation, the Ministry of Emergency Management said on Saturday.