CHINA yesterday expressed its willingness to co-operate with Russia in combatting the growing problem of Russian women working as prostitutes on the mainland. Foreign Ministry spokesman Mr Wu Jianmin said: ''We are willing to co-operate with other countries in suppressing this phenomenon. ''Prostitution is illegal in China. Once cases are discovered, the relevant departments will crack down on them,'' he said. Mr Wu did not specify what measures the Chinese Government was taking, or would be willing to take. Russian diplomats are said to have repeatedly voiced their concern to the Chinese Foreign Ministry over what many consider amounts to the ''slave labour'' of young Russian women in China who have been forced or tricked into prostitution. The Russian consulate in Shenyang, responsible for northeast China, which has the greatest concentration of Russian migrants, reportedly sent a formal letter to the Foreign Ministry last September warning that the importing of women for prostitution could ''seriously affect'' bilateral relations. Many Russian women, unable to find work at home, arrive in China after being offered employment in hotels, restaurants, nightclubs and karaoke bars. Only after taking up their jobs do they discover they are expected to work as prostitutes. Women who refused to work as prostitutes were sometimes prevented from leaving and forced into prostitution, European diplomats said. Other women, including some who went to China as students, had turned to prostitution after running out of money, the diplomats said. Most Russian prostitutes work in northeast China but many operate in Beijing, Shanghai and Guangzhou.