China accused of sending four ships and also increasing air operations in disputed East China Sea waters.

Chinese ships have entered waters near a group of disputed islands for the first time in three weeks, prompting a strong protest from Japan, which says China's air force has also sharply increased its operations in the area.

Japan's Coast Guard said the four Chinese surveillance ships were spotted within a 22km zone that Japan considers its territorial waters near one of the disputed islands in the East China Sea early on Thursday morning.