A Chinese surveillance vessel entered in Japan's territorial waters off the southwestern prefecture of Kagoshima early on Wednesday, a Japanese government spokesman said.
A Japanese patrol plane saw the Chinese vessel around 3:30 a.m local time, the Deputy Chief Cabinet Secretary, Hiroshige Seko, said. The ship left around 5 a.m., he added.
"The government will continue to take all possible measures for warning and surveillance activity for our territorial waters and airspace," Seko said, according to Reuters.
The Chinese ambassador was summoned last week by Japan after a Chinese navy ship sailed close to Japan's territorial waters in the East China Sea. It was the first time a Chinese ship entered the waters claimed by Japan, raising the tensions over the disputed area.