The stockpiles of Chinese coal producers have surged as economic woes home and abroad have dampened market demand, official data showed on Sunday.

Their stocks soared 70 percent from a year earlier to 98 million tons as of the end of September, according to figures from the National Development and Reform Commission, the country's top economic planning body.

Meanwhile, coal stocks at major ports rose 53 percent year-on-year to 40.76 million tons as of the end of September, the figures showed.

The increase was attributed to declining amount of coal shipments. Coal shipments by rail fell 0.1 percent from a year earlier to 1.68 billion tons in the first nine months, while waterway coal shipments dropped 6.4 percent to 460 million tons during the period, the figures showed.

Coal imports also slumped on the back of weak production demand. China imported 203 million tons of coal in the January-September period, up 36.3 percent year-on-year. But the growth rate has contracted for six straight months, according to the figures.

China's third-quarter growth has slowed to a seven-month low of 7.4 percent due to external jitters and government's tightening efforts to contain inflation.