A U.S. nuclear-powered submarine made a port call in Okinawa on Monday morning without giving the agreed prior notice to Japanese authorities, prompting Japan to lodge a protest and demand the U.S. Navy take measures to prevent any recurrence of such an incident, the Japanese Foreign Ministry's top diplomat said.

"I have been told that the U.S. side said there has been insufficient communication (within the U.S. Navy) and we have strongly demanded that measures be taken to ensure this will not happen again," Vice Foreign Minister Mitoji Yabunaka told a news conference, adding that 24-hour monitoring by Japanese authorities found no abnormalities with regard to radioactive leakages. The unannounced port call by the Los Angeles-class attack submarine Providence at White Beach in Uruma of Okinawa Prefecture took place between 10 a.m. to around noon, the ministry said. The U.S. Navy has a nuclear submarine port at its White Beach Naval Facility.