An explosion ripped through a railway tunnel that was under construction in Guizhou province on Tuesday, injuring 12 people and leaving 11 others trapped, local authorities said.

The incident happened around 2:50 pm in Liulong township, Dafang county, a spokesman for the county government said.

Explosive methane gas is suspected because methane is a byproduct of coal and the Qishanyan Tunnel passes through a coal seam, the spokesman said. The precise cause is under investigation.

The local government in Dafang has mobilized a rescue team consisting of firefighters,police and medical staff and sent it to the site.

However, rescue work has been hampered by high carbon monoxide levels, the authorities said.

By 7 pm, one of the trapped workers, who was injured, had been rescued and sent to the hospital, according to a report by China Central Television.

Authorities are still checking the identities of the trapped workers.

The geological condition of the mountain through which the tunnel passes is said to be complex. The risk of more explosions adds to the difficulty of ongoing rescue work.

The tunnel is part of a high-speed railway project linking Chengdu, Sichuan province, with Guiyang, Guizhou province.