China's heritage bureau has launched a probe into Chinese mining companies alleged to have brought down part of the Great Wall to allow their trucks to avoid paying road tolls, state media reported on Thursday.

Coal mining companies operating near Hujiayao village, on the border between the northern province of Shanxi and Inner Mongolia, had also taken soil from parts of the Ming-era (1368-1644) wall to build houses and piled "small mountains" of coal against it, Xinhua news agency reported, citing a local newspaper.

"Big trucks carrying coal had even opened a big gap in the Great Wall to make a coal shipping thoroughfare," the paper said.

The cultural heritage office in Datong, a major industrial centre in northern Shanxi, had declared the damage illegal and would investigate, the paper added.

Last October, China fined an investment company 500,000 yuan ($65,000) for building a highway through a section of the wall in Inner Mongolia, state media reported.

Snaking across more than 6,400 km (4,000 miles), the Great Wall receives an estimated 10 million visitors a year, mostly to the mere 10 km opened to tourists at Badaling, the nearest stretch to Beijing.

The United Nations listed it as a World Heritage site in 1987. It has been rebuilt many times through the centuries, and many sections have been destroyed permanently through natural erosion and nearby residents scrounging for building materials.