BEIJING, Sept. 19 (Xinhua) -- Japan's unlawful "purchase" of China's Diaoyu Islands will certainly affect and damage the development of economic and trade relations between the two countries, the Ministry of Commerce (MOC) said Wednesday.

"It is not what we wish to see, for which Japan should take full responsibility," MOC spokesman Shen Danyang said at a regular press conference.

The Diaoyu Islands have been China's inherent territory since ancient times, Shen said. Historical documents showed that the islands appeared on China's map since the Ming Dynasty (1368-1644), more than 400 years before Japan claimed discovery of the islands in 1884.

"The farce of buying the Diaoyu Islands seriously violates China's territorial sovereignty and severely hurts the feelings of the Chinese people, which have aroused strong indignation and opposition across the nation," the spokesman said.

Protests against Japan's "purchase" of the islands have been seen across China over the past few days. Many Japanese enterprises have suspended production as agitated Chinese people staged protests to boycott Japanese goods.

Shen voiced support to rational patriotic activities, saying that the ministry will firmly oppose illegal behaviors involving smashing and looting during protests.

Legitimate interests of foreign companies are under the protection of Chinese laws, Shen said, adding that foreign companies should seek help from local authorities when encountering offences.

The dispute over the islands has already cast a shadow over Sino-Japan trade, as trips to Japan were largely canceled at Chinese travel agencies. Sales of Japanese products, especially automobiles and electronics, have dropped sharply in China during the past month.

China ranks as Japan's largest trading partner, with the latter being China's fourth largest. Trade between the two countries accounts for about 20 percent of Japan's total foreign trade.