A huge area of China's soil covering more than twice the size of Spain is estimated to be polluted, the government said Thursday, announcing findings of a survey previously kept secret.

Of about 6.3 million square kilometres (2.4 million square miles) of soil surveyed -- roughly two thirds of China's total area -- 16.1 percent is thought to be polluted, the environmental protection ministry said in a report.

The study, which appeared on its website, blamed mining and farming practices among other causes.

"The national soil pollution situation is not positive," the ministry said, adding that more than 19 percent of the farmland which was surveyed is polluted.

The ministry last year described the results of its soil pollution survey as a state secret and refused to release the results, a move which incensed environmental campaigners.

The government has come under increasing pressure in recent years to take action to improve the environment, with large parts of the country repeatedly blanketed in thick smog and waterways and land polluted.

In response to public pressure, China has released more accurate data about air pollution.

More than 80 percent of the soil pollution was caused by "non-organic contaminants", the ministry said in its report. The survey was carried out over an eight-year period from 2005 to 2013.

The ministry last year acknowledged the existence of "cancer villages", years after Chinese media first reported on more than 100 polluted rural areas with a higher incidence of the disease.

Premier Li Keqiang announced in March that Beijing was "declaring war" on pollution, as he sought to address public concerns on issues ranging from smog to food safety, but experts warn that implementation will be the key.