Toxic gas following a mine blast has killed nine people and injured 12 others in China’s Yunnan province, authorities said on Sunday.

The accident happened at around 2:30 p.m. on Saturday in Lianmeng village, Kunming city, when 21 people were trapped under a mine filled with carbon monoxide after a blast, government officials said, Xinhua reported.

Two people were killed on the spot, while seven died after medical efforts failed. The injured are receiving treatment in hospital.

An investigation into the incident has been launched.