A bridge on the verge of completion in south China has collapsed, and the death toll rose to 31 Wednesday, with 28 others still missing, officials said.

The 328-meter (1,076-feet) bridge over the Tuo river in Hunan province crumbled on Monday as workers were removing steel scaffolding erected during the building work, the State Administration of Work Safety said.

At least 123 workers were at the construction site when the bridge collapsed, it said in a statement. Of those, 64 escaped or were rescued from the rubble.

"The search and rescue action is still going on at the scene," the statement said, adding that 22 survivors were being treated in hospital, two of whom are in critical condition.

State-run Xinhua news agency quoted an unofficial death toll of 34 as of early Wednesday.

The work safety administration gave no reason for the collapse, but China Central Television said initial investigations showed the structure was unstable.

China's Prime Minister Wen Jiabao warned that those found responsible for the disaster would face the full force of the law, according to the China Daily.

China's official Xinhua news agency said the manager and supervisor of the bridge project, which cost around $1.6 million, were being held by police.

The bridge was built by Hunan Provincial Road and Bridge Construction Company, a firm owned by the Hunan provincial government, the statement added.

The collapse came as the government announced plans to fix or rebuild about 6,300 damaged or shoddily constructed bridges across the country by 2010, according to Agence France-Presse.