Diplomatic tension was high Sunday between China and Japan over Japanese activists occupying an island both countries claim as theirs. Ten Japanese activists swam to the uninhabited island in the chain Japan calls the Senkaku Islands and China calls the Diaoyu Islands, The Wall Street Journal reported. The Chinese foreign ministry issued a warning over sovereignty. \"The Japanese side should properly handle the current issue and avoid seriously damaging the overall situation of China-Japan relations,\" the statement said. China\'s state-run Xinhua news agency said some 150 Japanese activists from \"right-wing groups\" were seen in boats around the island and several of them swam ashore. Last week, 14 Chinese activists from Hong Kong, Macau and China staged a similar occupation of the island. Xinhua said the island chain\'s ownership has become a hot issue in China, with rallies popping up throughout the country. At least 100 people demonstrated outside the Japanese Consulate in Guangzhou Sunday morning.