At least 33 people have died after being suffocated by carbon monoxide at a zinc mine in central Nigeria, witnesses have told the BBC.

The accident occurred at a mine run by mining company Solid Unity Nigeria, just outside the town of Wase.

The gas was believed to have accumulated in a mine shaft and the victims died before the end of their night shift.

The miners were believed to have died as the morning shift was about to start work, and more than 20 people were rescued and taken to hospital.

Security forces have sealed off the mine and are investigating how the toxic gas could have accumulated in the mine shaft.

State authorities have not yet visited the site of the accident due to security concerns, and emergency workers have been slow to respond.

Armed robberies have been active in the area in recent years.

Safianu Haruna, who arrived at work this morning to find his colleagues dead, told the BBC that some of the miners in the mine shaft survived and were taken to a hospital in Wase.