Vietnamese rescue workers battled Tuesday to pump air into a collapsed tunnel in a bid to save at least 11 people trapped in a construction site accident at a hydropower plant.

Rescuers were on an “extremely urgent” mission to drill a small hole to pump oxygen into the tunnel, local official Vo Van Hoang told AFP.

“Our drilling work is very slow and difficult, but our rescue operation is extremely urgent,” he said.

“The most important thing now is to pump oxygen into the tunnel to keep them alive, but we don’t know when we can introduce our oxygen tube,” he added.

Workers are also attempting to clear piles of sand, earth and stone, he said.

Hoang said the collapse at the tunnel – part of the Da Dang-Da Chomo hydroelectric power plant development in Lac Duong district of the Central Highlands – happened early Tuesday after heavy rain in the area.

“We mobilised more than 200 people to rescue those trapped inside but we have not been able to contact them,” he added.

Local media reported that the collapse happened around 500 meters into the tunnel.

Vietnam draws more than one-third of its electricity from hydropower.

Fatal collapses and rockslides are reasonably common at illegal stone quarries and mines in Vietnam due to lax regulations and safety standards.

But there have been few fatal incidents at the country’s many hydroelectric power plants.