Japanese foreign minister has summoned China’s ambassador to lodge a protest over the increased number of Chinese vessels in waters near disputed islands in the East China Sea.

The move by Fumio Kishida on Tuesday comes as the number of Chinese coast guard ships around the islands has nearly quadrupled over the past few days.

China’s coast guard vessels routinely sail around the islands, usually two to four at a time. But since last weekend, the number surged to a fleet of 13, several of them armed, according to the Japanese coast guard.

Kishida told Chinese Ambassador Cheng Yonghua that the ships must leave the area, saying their presence has escalated tensions.

China also claims Japan-controlled Senkaku islands and calls them the Diaoyu islands.