A Chinese military vessel has reportedly entered the East Sea as South Korea and the U.S. are engaged in a joint naval exercise in the sea. 

Japan’s defense authorities on Monday disclosed the current whereabouts of an intelligence-gathering ship belonging to the Chinese military, which was initially observed at around 100 kilometers southwest of Tsushima Island last Friday. 

It then passed through the Korea Strait before entering the East Sea, which Japan calls the Sea of Japan, the Japanese defense ministry said. 

It said three vessels of the Japan's Maritime Self-Defense Force were dispatched to collect related intelligence. 

Japan’s public broadcaster NHK quoted the defense ministry as saying that it is highly likely that the Chinese ship was sent to collect intelligence regarding the four-day South Korea-U.S. maritime exercise which launched on Monday. 

With Tokyo yet to reveal more details, the possibility of Japanese military vessels entering the East Sea under the pretext of monitoring the Chinese vessel may also draw attention from Seoul authorities.