China’s Zijin Mining Group Co. said it stopped operations at one of its mines in Tibet amid a hunt for six people missing after an accident.

A search and rescue operation is underway “in full force” at the Julong Copper and Polymetallic mine after the fall of a lift cage on Sunday, Zijin said in a statement to the Hong Kong stock exchange. Contact has been lost with six workers employed by a sub-contractor, Zijin said.

The mine is one of three run by Tibet Julong Copper Co., a 50.1%-owned subsidiary of Zijin that produces a total of 160,000 tons of copper a year.

The lift incident happened during the construction of a drainage system and the cause is under investigation, according to Zijin’s statement.