Chinese ships entered waters near the disputed Diaoyu Islands for the first time in three weeks Thursday.
Japan says China's air force has also sharply increased its operations in the area.
The ships refused to leave, saying the area was Chinese territory, according to Atsushi Takahashi, a spokesman for the Coast Guard's headquarters in Okinawa. He said it was the first time Chinese ships had entered the territorial waters since Oct. 3.
The island dispute has heightened tensions between the nations in recent months.
Both countries have been flexing their military muscles to bolster their claims to the islands, which are strategically located and surrounded by rich fishing grounds, along with potentially lucrative reserves of undersea natural resources.
Last week, China held exercises involving 11 vessels from its East China Sea fleet and the civilian marine surveillance and fishery administration agencies, along with eight aircraft, state media said.