BEIJING -- China's work safety watchdog on Thursday vowed to close over 1,700 accident-prone small coal mines in a bid to enhance safety and reduce casualties.

Yang Dongliang, head of the State Administration of Work Safety, said collieries with an annual capacity under 90,000 tons will be shut down in 50 coal-rich counties in 12 provincial-level regions.

But he did not give a specific timetable of the shutdown.

In the first half of 2014, 51 coal mine accidents in these counties killed 122 people. Small collieries in China usually have outdated safety-control measures and are vulnerable to flooding or gas explosion.

Yang said work safety authorities must step up supervision and hold local governments accountable for coal mine accidents.