China's largest producer of coal said on Monday it produced 13.3 percent more coal last month than a year before, or 15.3 million tonnes, and sold 19.7 million tonnes, sustaining the breakneck pace of increase in April.

Coal shortages along with surging prices, have pressured Chinese power generators and threaten to trigger outages as the country heads into the peak summer season. Stockpiles at key power plants in a number of provinces have fallen below the state-set warning level of seven days.

Official data released on Monday showed output hit a monthly record in May after the country's central government, worried shortages could lead to another round of economy-dampening brownouts this summer, urged more production.

Beijing is now encouraging its largest miners such as Shenhua and industry runner-up China Coal to hoard the fuel, while quickening the pace of approvals for smaller mines across the country to return to operation.

Shenhua's exports slid 4.8 percent to just 2 million tonnes.

The state-run company has said it planned to keep 2008 exports flat, holding back the fuel which fires three-quarters of the country's power capacity, for use at home.