At least 10 persons were killed in two separate incidents at neighbouring gold mines in China's Henan province, authorities said on Saturday.

Thick smoke engulfed a pit at Qinling gold mine of China National Gold Group in Lingbao city on Friday, trapping 12 workers and six management staff, Xinhua news agency cited a statement as saying.

Rescuers retrieved seven dead bodies. Of the 10 people who were sent to hospital, one failed to respond to emergency treatment and the other nine were recuperating.

One of the trapped workers remained missing as of Saturday morning.

The search and rescue operation had to be halted in the pit, where carbon monoxide density was extremely high and visibility was less than one metre.

In another accident in a neighbouring gold pit, six workers were trapped on Friday. Four workers were rescued and the other two died.

An investigation has been launched into the incidents.