At least 33 miners have died in a suspected carbon-monoxide leak at a lead and zinc mine in central Nigeria's Plateau state, witnesses have told the BBC.

The tragedy is believed to have occurred just before sunrise at a site outside the town of Wase, operated by the mining company Solid Unity Nigeria Ltd.

Toxic gas is believed to have built up underground in poorly ventilated tunnels, causing the workers to collapse just before the end of their night shift.

They were discovered by those reporting for work in the morning; more than 20 other miners were rescued and rushed to the hospital for treatment.

Security personnel have sealed off the mine, about 200km (124 miles) south-east of the state capital, Jos, as investigations begin to determine the cause of the leak.

State officials are yet to visit the area, and response efforts have reportedly been slow because of security concerns - armed criminal gangs, known locally as bandits, have been active there in recent years.

Safiyanu Haruna, one of the miners who found the bodies at the start of his shift, told the BBC that some of the miners who were underground survived and were taken to a hospital in Wase for treatment.

According to Haruna, the incident occurred at around 06:30 local time (05:30 GMT), killing 37 miners.

He said the miners had just finished their early-morning Muslim prayers and had returned underground to finish their shift.

The Plateau state government has issued a statement saying that, according to its preliminary investigation, 33 miners were killed in a blast at the mine - but workers at the scene say this is not the case.

"It was carbon-monoxide gas that leaked and killed them," Haruna said.

"There was no rescue for them at the time because those who were coming for the morning shift had yet to arrive," he said.

"It is sad to lose 37 miners who were struggling to make ends meet. We're disturbed by the incident."

The victims, believed to be men aged between 20 and 40, were buried shortly afterwards in accordance with local tradition, another local resident told the BBC.

The news has devastated the mainly Muslim community.

Mining disasters occur relatively frequently in Nigeria.

Less than two years ago, dozens of gold miners died after being trapped underground when a pit collapsed in neighbouring Niger state.

Officials believe the incident was caused by torrential rains that softened the soil.

The tragedy is likely to renew concerns over safety standards in Nigeria's mining sector.