FOREIGN and Hong Kong visitors to China have been warned against obtaining and smuggling confidential documents out of the country, according to an official report. A spokesman for the State Secrets Bureau (SSB) of the Shenzhen Municipal Government said the number of cases reported by Customs officials at border cities was growing. Last year, Kowloon Customs reported more than 49 cases to the Shenzhen SSB, nearly double 1992's figure. The semi-official Hong Kong China News Agency said yesterday non-mainland businessmen and Chinese government officials on business trips to Hong Kong or abroad were major couriers of confidential documents. The documents were mostly economic and trade papers. The agency said Lowu Customs reported more than 20 cases in the first three months of this year, including six cases in one week alone last month. A total of 100 confidential documents were recovered by Customs. The SSB spokesman stressed it was illegal to take information and confidential documents out of China without the approval of the bureau. He said people caught committing the crime faced legal action. He called on mainland officials and non-mainlanders to respect China's secrets laws. Although there are legal explanations of the different kinds of ''secrets'', critics say they are vague. Criticism has grown following the 12-year jail sentence imposed on Hong Kong journalist Xi Yang charged with ''stealing state financial secrets''.