Japan on Thursday continued tracking an unidentified submarine that entered its territorial waters off Okinawa the previous day.

Vice Defense Minister Takemasa Moriya said of the submarine, "It is navigating at an extremely slow speed and is not necessarily moving in one direction."

He said it is highly possible the sub is nuclear-powered because it has spent a long time submerged.

Only the United States, Russia and China have nuclear-powered subs in waters around Japan, he said.

A senior Foreign Ministry official said Japan must consider the political implications of the vessel's nationality.

"The Japan-China relationship is at a sensitive stage," he said, even though the sub has not yet been formally identified.

Media reports have quoted government sources as saying the sub is Chinese.

The Defense Agency said the submarine was in international waters west of Okinawa as of Thursday afternoon.

Moriya said the agency would continue tracking the sub until its destination becomes clear.

A senior agency official said the sub's slow speed and erratic course could mean it's testing the patrol and tracking capabilities of the Maritime Self-Defense Force.

China aware, mum

BEIJING (Kyodo) China said Thursday it is closely following the intrusion of an unidentified submarine into Japanese waters off Okinawa, but did not comment on whether it was Chinese, as suspected.

"China is now paying close attention to this issue," Foreign Ministry spokeswoman Zhang Qiyue said at a regular press briefing.

Asked if there were any Chinese submarines in the area, she said, "I have no information."