But scholars say unresolved issues will continue to hamper co-operation Japanese and Chinese leaders called for stronger ties yesterday - the 25th anniversary of the implementation of a peace treaty - but scholars said unresolved historical issues would continue to be a major setback in relations. In a letter to Premier Wen Jiabao, Japanese Prime Minister Junichiro Koizumi said the treaty of peace and friendship signed on August 12, 1978, 'played an important role as a basis of bilateral relations'. He added that Japan was ready to further develop the relationship and expand co-operation. And in a message to Mr Koizumi, Mr Wen said both countries had made efforts to develop friendly and co-operative relations since the treaty was signed, but he reminded Japan of the importance of dealing with historical issues between them. Although trade between the two countries has been rising in recent years, incidents that reminded China of Japan's war crimes have fuelled anti-Japanese sentiment on the mainland. The chemical weapons leak in Qiqihar, Heilongjiang province, which killed one and injured dozens in August, as well as a recent scandal in Zhuhai involving a party of Japanese hiring hundreds of mainland prostitutes, also fanned anti-Japanese feelings. This week, President Hu Jintao told Mr Koizumi at the Apec forum in Bangkok that Japan must remove all chemical weapons left on the mainland by the Japanese army. The Japanese government has promised to pay 300 million yen (HK$21.2 million) to clean up weapons left in Qiqihar, but stopped short of issuing a formal apology. Critics said the lack of an invitation from Mr Hu to Mr Koizumi to visit China was an indication that he was not pleased with the Japanese leader's repeated visits to the Yasukuni Shrine in Tokyo, which honours war criminals, among others, who were killed in the second world war. Zhang Yong, a researcher at the Institute of Japanese Studies at the Chinese Academy of Social Sciences, said Chinese suspicion and distrust for Japan had become the biggest thorn in the countries' relationship. With a growing civil society, an increasingly influential media and the internet, public opinion would have a bigger impact on Chinese politics, which would influence leaders' attitudes towards Japan, he said. Mr Zhang said Japan and China should look to their common interests, such as economic and security issues, to work towards a closer relationship. But Ken Wang, a political scientist at the University of Hong Kong, said while public opinion might be influential in Chinese politics, it was unlikely to reverse the trend of the spirit of co-operation between the two countries.