BEIJING - Flagging demand in China has dented output at the country's top two coal firms, as slower economic growth took a toll on the world's largest producer and consumer of the mineral.

China Shenhua Energy Co, Ltd, China's largest coal miner, reported on Tuesday that its coal output declined 10.1 percent year on year to 139 million tonnes in the first half of 2015.

China Coal Energy Co., Ltd, the second-largest coal producer, said on the same day its output slumped 22.1 percent to 46.3 million tonnes in the first six months.

During that period, coal sales by China Shenhua Energy slipped 24.2 percent and those by China Coal Energy fell 12 percent, according to the two companies' statements.

The drop in sales was caused by weaker demand, the impact of government measures to reduce pollution and other reasons, China Shenhua Energy said.

The coal industry will remain anemic in the short term and it will get more difficult for producers to survive, said Wang Xianzheng, head of the China National Coal Association.

Coal accounts for about 66 percent of China's primary energy consumption, 35 percentage points higher than the world average.

The country's coal output fell in 2014 for the first time this century as a result of slowing economic growth, government efforts to reduce air pollution and increased investment in renewable energy.

China's economy expanded 7 percent year on year in the second quarter, unchanged from the first quarter but still much slower than the previous double-digit growth.

Overall coal production in China slipped 6.1 percent in the first four months this year to 1.15 billion tonnes, with the pace of decline accelerating from a 3.5-percent fall registered in the first three months of the year, official data revealed earlier.