Gan said in a statement that several Japanese patrol boats "illegally entered China's territorial waters in the Diaoyu Islands area" on Monday.
"[We] are calling on Japan to immediately cease all illegal activities in the said maritime area and ensure that similar incidents do not recur," he said.
The spokesman added that Chinese coast guard vessels gave the Japanese ships a legal warning to leave the waters. Gan reiterated that the Diaoyu Islands are an inalienable territory of China, and Chinese coast guard carries out maritime rights protection and law enforcement activities in the area in accordance with the law.
Japan claims to have had sovereignty over the islands since 1895, while China argues that they were marked as Chinese territory on Japanese maps dated 1783 and 1785.
The United States took control over the islands during WWII and handed them over to Japan in 1972. China says that the islands were seized illegally by Japan, while Japan claims that China only became interested in them after shale oil was discovered under the seabed. The territorial dispute escalated in 2012, when the Japanese government purchased three of the five islands from a private owner, thereby confirming Japan's affiliation with the islands.