Tokyo lodged a strong protest with Beijing on Saturday, after a Chinese Navy survey ship entered Japanese territorial waters off Kagoshima Prefecture — just days following the first foray into Japanese airspace by a Chinese military aircraft.

The Japanese Defense Ministry said the Chinese Shupang-class vessel’s entry into the territorial waters southwest of Kagoshima’s Kuchinoerabu Island was the 13th time since last September that the Chinese Navy had entered Japanese waters, with the ministry calling the moves “cause for concern.”

The Chinese ship, which entered the waters around 6 a.m. before departing nearly two hours later, was monitored by Maritime Self-Defense Force vessels and aircraft.