Hanoi  - Engineers have discovered two new cracks in the body of a 130-metre-high dam under construction in the province of Son La, Vietnamese media reported Wednesday.

The newspaper Thanh Nien (Youth) reported the cracks were in addition to another one discovered in September 2008. The largest is 31.5 metres long and 1 millimetre wide, and runs 6 metres deep into the body of the dam.

The paper reported Nguyen Kim Toi, director of the construction project, said the cracks could be due to shrinkage following temperature changes when the concrete encountered rain.

"Compared with the block of concrete it is in, the 6-metre deep crack is fairly big," the paper quoted Toi saying. "But if you compare it with all of the concrete in the Son La dam, it is not big."

Toi said the cracks had been isolated with steel bars, and that outside consultants had determined they were not dangerous.

The Son La dam is the largest ever built in Vietnam, 90 metres wide and measuring over 1 kilometre in length. When complete in 2015 it is expected to generate 9.4 billion kilowatt hours of electricity per year.

The dam has been controversial since it was first proposed. Some 91,000 people, mainly ethnic minorities, must be evacuated from the basin of the Da River, which will be flooded. The province of Son La is prone to earthquakes, and if the dam fails, it could flood the city of Hanoi. (dpa)