Coast Guard boats were on patrol in waters around Japan and would issue warnings via radio to any South Korean ships entering territory claimed by Japan, Coast Guard spokesman Takatoshi Nagasaki said.
Nagasaki refused to elaborate on where the boats were patrolling, or if Japan planned to increase the number of patrol boats because of the survey announced by Seoul.
The comments came after a top Foreign Ministry bureaucrat Shotaro Yachi told reporters that a maritime survey around the disputed islets was one of a range of options Tokyo was considering in response to South Korea's survey.
"I believe (a survey) is one option available to us," Yachi said.
Japan and South Korea have long sparred over ownership of the islets, called Dokdo in Korean and Takeshima in Japanese, as part of a wider struggle to define their respective exclusive economic zones, the AP reports.
To understand how China will act, one must understand the logic of China's development. This logic has always been almost the same, be it the Middle Ages, or modern times