QUETTA, Pakistan (Reuters) - A mine explosion in Pakistan’s southwestern city of Quetta killed at least seven coal miners, a mining official said on Monday, with authorities expecting the death toll could rise.

The bodies of the dead were recovered on Monday morning, with efforts being made to retrieve six more men trapped in the mine but feared dead.

“The explosion was caused by accumulation of methane gas,” Iftikhar Ahmed, the chief inspector of mines in Baluchistan, the province home to Quetta, told Reuters. “The miners were working at the depth of 400 feet at the time.”

The figure is equivalent to 122 m.

Explosions are not uncommon in Pakistan’s coal mines, most of which are in Baluchistan and the neighboring province of Sindh, where safety measures can be lax.

In May, more than 20 miners were killed in a mine blast in Quetta.

Pakistan has huge reserves of coal, estimated at more than 184 billion tonnes. It produces 4 million tonnes of coal annually, most of which is consumed by brick-making kilns.