China has vowed harsh penalties for foreigners conducting illegal surveys in China, months after deporting two Japanese scholars doing unauthorised research in the remote far west, state media said on Wednesday.

The Japanese collected materials and took coordinates of an airport and water facilities in the mostly Muslim Xinjiang autonomous region, Xinhua news agency said at the time.  

The two were fined a total 80,000 yuan ($10,000) and deported, Xinhua said, adding that the equipment they were using was "more precise than that which normal tourists would use" and the results could be put to military use, it said.

It did not elaborate.

"Foreigners who carry out unlawful geographic mensuration and collection and expose the important statistics without permission will face severe punishment," the Beijing News said, citing China's State Bureau of Surveying and Mapping.

Foreigners were also forbidden to take away the results out of research which involve "secrets" or put them on Internet, it added.