A Chinese bridge that collapsed killing at least 41 people was made with rocks and concrete rather than stronger steel so that it would be in "harmony with the environment", state media said, quoting an official.

The death toll from Monday's collapse of the almost-completed Fenghuang highway bridge in the southern province of Hunan was expected to rise further. Dozens of construction workers were feared trapped beneath a huge pile of debris in the Tuo river.

"While the cause of the collapse is still unknown, a local official at the scene claimed that a 'traditional and risky' model of bridge, made of stone and concrete, had been chosen over a steel structure to ensure it remained 'in harmony with the natural environment'," the China Daily said.

Investigation teams and experts have pointed a finger of blame at design flaws and quality defects in the 42-metre (140 feet) high bridge's structure and materials.

"The ruptured parts of the bridge show broken stones, also it was a clean break. It's obvious the quality was too poor," the Beijing News quoted an architecture expert at Beijing's Tsinghua university as saying.

Surviving construction workers said work on the bridge had been rushed and that they had been ordered to take scaffolding down before the bridge was finished.

"Beforehand, we said that such a poor-quality stone arch bridge would one day have an accident. We never thought it would topple before it was finished," the paper quoted a worker as saying.

Another said no safety harnesses were provided on scaffolding "tens of metres high" and that workers' safety helmets were so flimsy that holes could be poked in them with fingers.

China's top safety chief, Li Yuanzhong, leading a cabinet-appointed probe, said administrative failures were also behind the disaster.

"The painful lesson...is that acute safety dangers existed before the accident. Relevant work units and personnel did not inspect or detect (the dangers), let alone take effective measures," Li was quoted by Xinhua news agency as saying.

More than 1,500 rescue workers were picking through the debris to find the missing, Xinhua said, adding that President Hu Jintao had ordered authorities to severely punish those responsible.

Police had detained a construction manager and a project supervisor, Xinhua said on Tuesday, a day after the bridge broke apart like a pat of "beancurd", state media said, quoting a rescuer.

One of the contractors building the bridge also worked on a bridge that fell apart two months ago, local media reported on Thursday.

On June 15, a section of the 1,600-metre Jiujiang bridge, connecting Foshan and Heshan over the Xijiang river in neighbouring Guangdong province, collapsed after a barge laden with sand crashed into it, causing several cars and pedestrians to fall into the river below.

Investigators detained the captain and five other suspects. The bridge was deemed structurally sound.