HANOI (REUTERS) - Vietnamese rescuers battled on Wednesday to free 12 workers trapped in a tunnel that collapsed during construction of a hydropower plant and said it could take days to get past entrenched mud, sand and debris as water levels rose.

The rescue involves hundreds of firefighters and members of the security forces. They managed late on Tuesday to ram a pipe through the rubble to supply air to the workers as oxygen dropped to a critical level eight hours after the accident.

But bad weather and the complicated geology of the site in the Central Highlands province of Lam Dong were hampering the effort and authorities had a race against time to drain out water after levels rose to one metre.

They dug small tunnels and reinforced the site with tree trunks to pull out sand during a mission that has dominated television networks and the front pages. "Now there's only one tube to send food and oxygen inside, while water is rising," Dang Quang Dat, director of Song Da 505 , the workers' firm, told Vietnam Television.

"We can't just dig when we want."

The US$22 million (S$ 28 million) hydropower plant, one of dozens planned or under construction in energy-hungry Vietnam, was expected to produce about 110 million kWh annually.

An iron tube 60 centimetres in diameter was being used to pull out rocks and sand and authorities were pinning hopes on reaching the workers so they could eventually squeeze through the pipe to get out. "It's very difficult and narrow ... most work is being done by hand with the help of some equipment," provincial military chief Du Truong Giang told VTV.

It was not clear what caused the tunnel to collapse but a preliminary investigation suggested the cause was heavy rain.

Worker Nguyen Van Tuan said he was in a truck inside the 500 metre tunnel and fled as it caved in.

"Dirt, rocks and mud tumbled in ... we came back to dig out our trapped colleagues but dirt and rocks kept falling as we dug, so we had to get help," he told news the VnExpress website.