A cesspool filled with excrement has exploded in central China, injuring 15 people and knocking down a building, state-run media reported.

The blast was apparently sparked by a local man burning waste close to the cesspool, igniting methane gas which was emanating from the pit, the Xinhua news agency said late on Sunday.

The incident in Zhangjiajie city, in the central province of Hunan, caused a residential building to collapse and three of the injured had to be hospitalised, Xinhua said.

China's urban infrastructure has often been hastily built with little regard for safety as hundreds of millions of people have moved from the countryside to cities in recent decades.

In another excrement-related incident, a man and his mother both died trying to retrieve his wife's new mobile phone -- worth 2,000 yuan ($320) -- from a toilet septic tank, reports said in May.

The man jumped in to try to locate the phone but was overcome by fumes and passed out, according to Dahe, a web portal run by the provincial government in central Henan province.