A gas explosion has killed 18 miners at an unlicensed pit in China's south-western province of Guizhou, state media have reported.

Seven more miners were injured in the blast at the privately-run Taojiawan coal mine in Liupanshui.

An investigation has been launched into the accident, which brought the death toll for miners in China to more than 240 since June and about 3,500 for the year so far.

Earlier this month, the authorities told 10,000 miners to leave their jobs in a crackdown against illegal gold mines in northern Shanxi province following a string of high-profile disasters.

Correspondents say that central and local authorities publicise their efforts to close unlicensed mines and those which fail to pass safety tests.

But they add that many small mines ordered to close manage to re-open because local governments are hungry for tax revenues.